======================================================================
= PDF =
======================================================================
Introduction
======================================================================
Portable document format (PDF), standardized as ISO 32000, is a file
format developed by Adobe in 1992 to present documents, including text
formatting and images, in a manner independent of application
software, hardware, and operating systems. Based on the PostScript
language, each PDF file encapsulates a complete description of a
fixed-layout flat document, including the text, fonts, vector
graphics, raster images and other information needed to display it.
PDF has its roots in "The Camelot Project" initiated by Adobe
co-founder John Warnock in 1991.
PDF was standardized as ISO 32000 in 2008. The last edition as ISO
32000-2:2020 was published in December 2020.
PDF files may contain a variety of content besides flat text and
graphics including logical structuring elements, interactive elements
such as annotations and form-fields, layers, rich media (including
video content), three-dimensional objects using U3D or PRC, and
various other data formats. The PDF specification also provides for
encryption and digital signatures, file attachments, and metadata to
enable workflows requiring these features.
History
======================================================================
The development of PDF began in 1991 when John Warnock wrote a paper
for a project then code-named Camelot, in which he proposed the
creation of a simplified version of PostScript called Interchange
PostScript (IPS). Unlike traditional PostScript, which was tightly
focused on rendering print jobs to output devices, IPS would be
optimized for displaying pages to any screen and any platform.
Adobe Systems made the PDF specification available free of charge in
1993. In the early years PDF was popular mainly in desktop publishing
workflows, and competed with several other formats, including DjVu,
Envoy, Common Ground Digital Paper, Farallon Replica and even Adobe's
own PostScript format.
PDF was a proprietary format controlled by Adobe until it was released
as an open standard on July 1, 2008, and published by the
International Organization for Standardization as ISO 32000-1:2008, at
which time control of the specification passed to an ISO Committee of
volunteer industry experts. In 2008, Adobe published a Public Patent
License to ISO 32000-1 granting royalty-free rights for all patents
owned by Adobe necessary to make, use, sell, and distribute
PDF-compliant implementations.
PDF 1.7, the sixth edition of the PDF specification that became ISO
32000-1, includes some proprietary technologies defined only by Adobe,
such as Adobe XML Forms Architecture (XFA) and JavaScript extension
for Acrobat, which are referenced by ISO 32000-1 as normative and
indispensable for the full implementation of the ISO 32000-1
specification. These proprietary technologies are not standardized,
and their specification is published only on Adobe's website. Many of
them are not supported by popular third-party implementations of PDF.
ISO published version 2.0 of PDF, ISO 32000-2 in 2017, available for
purchase, replacing the free specification provided by Adobe. In
December 2020, the second edition of PDF 2.0, ISO 32000-2:2020, was
published, with clarifications, corrections, and critical updates to
normative references (ISO 32000-2 does not include any proprietary
technologies as normative references).
In April 2023 the PDF Association made ISO 32000-2 available for
download free of charge.
Technical details
======================================================================
A PDF file is often a combination of vector graphics, text, and bitmap
graphics. The basic types of content in a PDF are:
* Typeset text stored as content streams (i.e., not encoded in plain
text);
* Vector graphics for illustrations and designs that consist of shapes
and lines;
* Raster graphics for photographs and other types of images; and
* Other multimedia objects.
In later PDF revisions, a PDF document can also support links (inside
document or web page), forms, JavaScript (initially available as a
plugin for Acrobat 3.0), or any other types of embedded contents that
can be handled using plug-ins.
PDF combines three technologies:
* An equivalent subset of the PostScript page description programming
language but in declarative form, for generating the layout and
graphics.
* A font-embedding/replacement system to allow fonts to travel with
the documents.
* A structured storage system to bundle these elements and any
associated content into a single file, with data compression where
appropriate.
PostScript language
=====================
PostScript is a page description language run in an interpreter to
generate an image. It can handle graphics and has standard features of
programming languages such as branching and looping. PDF is a subset
of PostScript, simplified to remove such control flow features, while
graphics commands remain.
PostScript was originally designed for a drastically different use
case: transmission of one-way linear print jobs in which the
PostScript interpreter would collect a series of commands until it
encountered the showpage command, then execute all the commands to
render a page as a raster image to a printing device. PostScript was
not intended for long-term storage and real-time interactive rendering
of electronic documents to computer monitors, so there was no need to
support anything other than consecutive rendering of pages. If there
was an error in the final printed output, the user would correct it at
the application level and send a new print job in the form of an
entirely new PostScript file. Thus, any given page in a PostScript
file could be accurately rendered only as the cumulative result of
executing all preceding commands to draw all previous pages--any of
which could affect subsequent pages--plus the commands to draw that
particular page, and there was no easy way to bypass that process to
skip around to different pages.
Traditionally, to go from PostScript to PDF, a source PostScript file
(that is, an executable program) is used as the basis for generating
PostScript-like PDF code (see, e.g., Adobe Distiller). This is done by
applying standard compiler techniques like loop unrolling, inlining
and removing unused branches, resulting in code that is purely
declarative and static. The result is then packaged into a container
format, together with all necessary dependencies for correct rendering
(external files, graphics, or fonts to which the document refers), and
compressed. Modern applications write to printer drivers that directly
generate PDF rather than going through PostScript first.
As a document format, PDF has several advantages over PostScript:
* PDF contains only static declarative PostScript code that can be
processed as data, and does not require a full program interpreter or
compiler. This avoids the complexity and security risks of an engine
with such a higher complexity level.
* Like Display PostScript, PDF has supported transparent graphics
since version 1.4, while standard PostScript does not.
* PDF enforces the rule that the code for any particular page cannot
affect any other pages. That rule is strongly recommended for
PostScript code too, but has to be implemented explicitly (see, e.g.,
the Document Structuring Conventions), as PostScript is a full
programming language that allows for such greater flexibilities and is
not limited to the concepts of pages and documents.
* All data required for rendering is included within the file itself,
improving portability.
Its disadvantages are:
* A loss of flexibility, and limitation to a single use case.
* A (sometimes much) larger file size.
PDF since v1.6 supports embedding of interactive 3D documents: 3D
drawings can be embedded using U3D or PRC and various other data
formats.
File format
======================================================================
A PDF file is organized using ASCII characters, except for certain
elements that may have binary content.
The file starts with a header containing a magic number (as a readable
string) and the version of the format, for example %PDF-1.7. The
format is a subset of a COS ("Carousel" Object Structure) format. A
COS tree file consists primarily of 'objects', of which there are nine
types:
* Boolean values, representing 'true' or 'false'
* Real numbers
* Integers
* Strings, enclosed within parentheses ((...)) or represented as
hexadecimal within single angle brackets (<...>). Strings may
contain 8-bit characters.
* Names, starting with a forward slash (/)
* Arrays, ordered collections of objects enclosed within square
brackets ([...])
* Dictionaries, collections of objects indexed by names enclosed
within double angle brackets (<<...>>)
* Streams, usually containing large amounts of optionally compressed
binary data, preceded by a dictionary and enclosed between the stream
and endstream keywords.
* The null object
Comments using 8-bit characters prefixed with the percent sign (%) may
be inserted.
Objects may be either 'direct' (embedded in another object) or
'indirect'. Indirect objects are numbered with an 'object number' and
a 'generation number' and defined between the obj and endobj keywords
if residing in the document root. Beginning with PDF version 1.5,
indirect objects (except other streams) may also be located in special
streams known as 'object streams' (marked /Type /ObjStm). This
technique enables non-stream objects to have standard stream filters
applied to them, reduces the size of files that have large numbers of
small indirect objects and is especially useful for 'Tagged PDF'.
Object streams do not support specifying an object's 'generation
number' (other than 0).
An index table, also called the cross-reference table, is located near
the end of the file and gives the byte offset of each indirect object
from the start of the file. This design allows for efficient random
access to the objects in the file, and also allows for small changes
to be made without rewriting the entire file ('incremental update').
Before PDF version 1.5, the table would always be in a special ASCII
format, be marked with the xref keyword, and follow the main body
composed of indirect objects. Version 1.5 introduced optional
'cross-reference streams', which have the form of a standard stream
object, possibly with filters applied. Such a stream may be used
instead of the ASCII cross-reference table and contains the offsets
and other information in binary format. The format is flexible in that
it allows for integer width specification (using the /W array), so
that for example, a document not exceeding 64 KiB in size may dedicate
only 2 bytes for object offsets.
At the end of a PDF file is a footer containing
* The startxref keyword followed by an offset to the start of the
cross-reference table (starting with the xref keyword) or the
cross-reference stream object, followed by
* The %%EOF end-of-file marker.
If a cross-reference stream is not being used, the footer is preceded
by the trailer keyword followed by a dictionary containing information
that would otherwise be contained in the cross-reference stream
object's dictionary:
* A reference to the root object of the tree structure, also known as
the 'catalog' (/Root)
* The count of indirect objects in the cross-reference table (/Size)
* Other optional information
Within each page, there are one or multiple content streams that
describe the text, vector and images being drawn on the page. The
content stream is stack-based, similar to PostScript.
There are two layouts to the PDF files: non-linearized (not
"optimized") and linearized ("optimized"). Non-linearized PDF files
can be smaller than their linear counterparts, though they are slower
to access because portions of the data required to assemble pages of
the document are scattered throughout the PDF file. Linearized PDF
files (also called "optimized" or "web optimized" PDF files) are
constructed in a manner that enables them to be read in a Web browser
plugin without waiting for the entire file to download, since all
objects required for the first page to display are optimally organized
at the start of the file. PDF files may be optimized using Adobe
Acrobat software or QPDF.
Page dimensions are not limited by the format itself. However, Adobe
Acrobat imposes a limit of 15 million by 15 million inches, or 225
trillion in2 (145,161 km2).
Imaging model
======================================================================
The basic design of how graphics are represented in PDF is very
similar to that of PostScript, except for the use of transparency,
which was added in PDF 1.4.
PDF graphics use a device-independent Cartesian coordinate system to
describe the surface of a page. A PDF page description can use a
matrix to scale, rotate, or skew graphical elements. A key concept in
PDF is that of the 'graphics state', which is a collection of
graphical parameters that may be changed, saved, and restored by a
'page description'. PDF has (as of version 2.0) 25 graphics state
properties, of which some of the most important are:
* The 'current transformation matrix' (CTM), which determines the
coordinate system
* The 'clipping path'
* The 'color space'
* The 'alpha constant', which is a key component of transparency
*'Black point compensation' control (introduced in PDF 2.0)
Vector graphics
=================
As in PostScript, vector graphics in PDF are constructed with 'paths'.
Paths are usually composed of lines and cubic Bézier curves, but can
also be constructed from the outlines of text. Unlike PostScript, PDF
does not allow a single path to mix text outlines with lines and
curves. Paths can be stroked, filled, fill then stroked, or used for
clipping. Strokes and fills can use any color set in the graphics
state, including 'patterns'. PDF supports several types of patterns.
The simplest is the 'tiling pattern' in which a piece of artwork is
specified to be drawn repeatedly. This may be a 'colored tiling
pattern', with the colors specified in the pattern object, or an
'uncolored tiling pattern', which defers color specification to the
time the pattern is drawn. Beginning with PDF 1.3 there is also a
'shading pattern', which draws continuously varying colors. There are
seven types of shading patterns of which the simplest are the 'axial
shading' (Type 2) and 'radial shading' (Type 3).
Raster images
===============
Raster images in PDF (called 'Image XObjects') are represented by
dictionaries with an associated stream. The dictionary describes the
properties of the image, and the stream contains the image data. (Less
commonly, small raster images may be embedded directly in a page
description as an 'inline image'.) Images are typically 'filtered' for
compression purposes. Image filters supported in PDF include the
following general-purpose filters:
* 'ASCII85Decode', a filter used to put the stream into 7-bit ASCII,
* 'ASCIIHexDecode', similar to ASCII85Decode but less compact,
* 'FlateDecode', a commonly used filter based on the deflate algorithm
defined in (deflate is also used in the gzip, PNG, and zip file
formats among others); introduced in PDF 1.2; it can use one of two
groups of predictor functions for more compact zlib/deflate
compression: 'Predictor 2' from the TIFF 6.0 specification and
predictors (filters) from the PNG specification (),
* 'LZWDecode', a filter based on LZW Compression; it can use one of
two groups of predictor functions for more compact LZW compression:
'Predictor 2' from the TIFF 6.0 specification and predictors (filters)
from the PNG specification,
* 'RunLengthDecode', a simple compression method for streams with
repetitive data using the run-length encoding algorithm and the
image-specific filters,
* 'DCTDecode', a lossy filter based on the JPEG standard,
* 'CCITTFaxDecode', a lossless bi-level (black/white) filter based on
the Group 3 or Group 4 CCITT (ITU-T) fax compression standard defined
in ITU-T T.4 and T.6,
* 'JBIG2Decode', a lossy or lossless bi-level (black/white) filter
based on the JBIG2 standard, introduced in PDF 1.4, and
* 'JPXDecode', a lossy or lossless filter based on the JPEG 2000
standard, introduced in PDF 1.5.
Normally all image content in a PDF is embedded in the file. But PDF
allows image data to be stored in external files by the use of
'external streams' or 'Alternate Images'. Standardized subsets of PDF,
including PDF/A and PDF/X, prohibit these features.
Text
======
Text in PDF is represented by 'text elements' in page content streams.
A text element specifies that 'characters' should be drawn at certain
positions. The characters are specified using the 'encoding' of a
selected 'font resource'.
A font object in PDF is a description of a digital typeface. It may
either describe the characteristics of a typeface, or it may include
an embedded 'font file'. The latter case is called an 'embedded font'
while the former is called an 'unembedded font'. The font files that
may be embedded are based on widely used standard digital font
formats: Type 1 (and its compressed variant CFF), TrueType, and
(beginning with PDF 1.6) OpenType. Additionally PDF supports the Type
3 variant in which the components of the font are described by PDF
graphic operators.
Fourteen typefaces, known as the 'standard 14 fonts', have a special
significance in PDF documents:
* Times (v3) (in regular, italic, bold, and bold italic)
* Courier (in regular, oblique, bold and bold oblique)
* Helvetica (v3) (in regular, oblique, bold and bold oblique)
* Symbol
* Zapf Dingbats
These fonts are sometimes called the 'base fourteen fonts'. These
fonts, or suitable substitute fonts with the same metrics, should be
available in most PDF readers, but they are not 'guaranteed' to be
available in the reader, and may only display correctly if the system
has them installed. Fonts may be substituted if they are not embedded
in a PDF.
Within text strings, characters are shown using 'character codes'
(integers) that map to glyphs in the current font using an 'encoding'.
There are several predefined encodings, including 'WinAnsi',
'MacRoman', and many encodings for East Asian languages and a font can
have its own built-in encoding. (Although the WinAnsi and MacRoman
encodings are derived from the historical properties of the Windows
and Macintosh operating systems, fonts using these encodings work
equally well on any platform.) PDF can specify a predefined encoding
to use, the font's built-in encoding or provide a lookup table of
differences to a predefined or built-in encoding (not recommended with
TrueType fonts). The encoding mechanisms in PDF were designed for Type
1 fonts, and the rules for applying them to TrueType fonts are
complex.
For large fonts or fonts with non-standard glyphs, the special
encodings 'Identity-H' (for horizontal writing) and 'Identity-V' (for
vertical) are used. With such fonts, it is necessary to provide a
'ToUnicode' table if semantic information about the characters is to
be preserved.
A text document which is scanned to PDF without the text being
recognised by optical character recognition (OCR) is an image, with no
fonts or text properties.
Transparency
==============
The original imaging model of PDF was 'opaque,' similar to PostScript,
where each object drawn on the page completely replaced anything
previously marked in the same location. In PDF 1.4 the imaging model
was extended to allow transparency. When transparency is used, new
objects interact with previously marked objects to produce blending
effects. The addition of transparency to PDF was done by means of new
extensions that were designed to be ignored in products written to PDF
1.3 and earlier specifications. As a result, files that use a small
amount of transparency might be viewed acceptably by older viewers,
but files making extensive use of transparency could be viewed
incorrectly by an older viewer.
The transparency extensions are based on the key concepts of
'transparency groups', 'blending modes', 'shape', and 'alpha'. The
model is closely aligned with the features of Adobe Illustrator
version 9. The blend modes were based on those used by Adobe Photoshop
at the time. When the PDF 1.4 specification was published, the
formulas for calculating blend modes were kept secret by Adobe. They
have since been published.
The concept of a transparency group in PDF specification is
independent of existing notions of "group" or "layer" in applications
such as Adobe Illustrator. Those groupings reflect logical
relationships among objects that are meaningful when editing those
objects, but they are not part of the imaging model.
Additional features
======================================================================
=== Logical structure and accessibility ===
A tagged PDF (see clause 14.8 in ISO 32000) includes document
structure and semantics information to enable reliable text extraction
and accessibility. Technically speaking, tagged PDF is a stylized use
of the format that builds on the logical structure framework
introduced in PDF 1.3. Tagged PDF defines a set of standard structure
types and attributes that allow page content (text, graphics, and
images) to be extracted and reused for other purposes.
Tagged PDF is not required in situations where a PDF file is intended
only for print. Since the feature is optional, and since the rules for
tagged PDF were relatively vague in ISO 32000-1, support for tagged
PDF among consuming devices, including assistive technology (AT), is
uneven as of 2021. ISO 32000-2, however, includes an improved
discussion of tagged PDF which is anticipated to facilitate further
adoption.
An ISO-standardized subset of PDF specifically targeted at
accessibility, PDF/UA, was first published in 2012.
Optional Content Groups (layers)
==================================
With the introduction of PDF version 1.5 (2003) came the concept of
Layers. Layers, more formally known as Optional Content Groups (OCGs),
refer to sections of content in a PDF document that can be selectively
viewed or hidden by document authors or viewers. This capability is
useful in CAD drawings, layered artwork, maps, multi-language
documents, etc.
Basically, it consists of an Optional Content Properties Dictionary
added to the document root. This dictionary contains an array of
Optional Content Groups (OCGs), each describing a set of information
and each of which may be individually displayed or suppressed, plus a
set of Optional Content Configuration Dictionaries, which give the
status (Displayed or Suppressed) of the given OCGs.
Encryption and signatures
===========================
A PDF file may be encrypted, for security, in which case a password is
needed to view or edit the contents. PDF 2.0 defines 256-bit AES
encryption as the standard for PDF 2.0 files. The PDF Reference also
defines ways that third parties can define their own encryption
systems for PDF.
PDF files may be digitally signed, to provide secure authentication;
complete details on implementing digital signatures in PDF are
provided in ISO 32000-2.
PDF files may also contain embedded DRM restrictions that provide
further controls that limit copying, editing, or printing. These
restrictions depend on the reader software to obey them, so the
security they provide is limited.
The standard security provided by PDF consists of two different
methods and two different passwords: a 'user password', which encrypts
the file and prevents opening, and an 'owner password', which
specifies operations that should be restricted even when the document
is decrypted, which can include modifying, printing, or copying text
and graphics out of the document, or adding or modifying text notes
and AcroForm fields. The user password encrypts the file, while the
owner password does not, instead relying on client software to respect
these restrictions. An owner password can easily be removed by
software, including some free online services. Thus, the use
restrictions that a document author places on a PDF document are not
secure, and cannot be assured once the file is distributed; this
warning is displayed when applying such restrictions using Adobe
Acrobat software to create or edit PDF files.
Even without removing the password, most freeware or open source PDF
readers ignore the permission "protections" and allow the user to
print or make copies of excerpts of the text as if the document were
not limited by password protection.
Beginning with PDF 1.5, Usage rights (UR) signatures are used to
enable additional interactive features that are not available by
default in a particular PDF viewer application. The signature is used
to validate that the permissions have been granted by a bona fide
granting authority. For example, it can be used to allow a user:
* To save the PDF document along with a modified form or annotation
data
* Import form data files in FDF, XFDF, and text (CSV/TSV) formats
* Export form data files in FDF and XFDF formats
* Submit form data
* Instantiate new pages from named page templates
* Apply a digital signature to existing digital signature form field
* Create, delete, modify, copy, import, and export annotations
For example, Adobe Systems grants permissions to enable additional
features in Adobe Reader, using public-key cryptography. Adobe Reader
verifies that the signature uses a certificate from an
Adobe-authorized certificate authority. Any PDF application can use
this same mechanism for its own purposes.
Under specific circumstances including non-patched systems of the
receiver, the information the receiver of a digital signed document
sees can be manipulated by the sender after the document has been
signed by the signer.
PAdES ('PDF Advanced Electronic Signatures') is a set of restrictions
and extensions to PDF and ISO 32000-1 making it suitable for advanced
electronic signatures. This is published by ETSI as TS 102 778.
File attachments
==================
PDF files can have file attachments which processors may access and
open or save to a local filesystem.
Metadata
==========
PDF files can contain two types of metadata. The first is the Document
Information Dictionary, a set of key/value fields such as author,
title, subject, creation and update dates. This is optional and is
referenced from an Info key in the trailer of the file. A small set of
fields is defined and can be extended with additional text values if
required. This method is deprecated in PDF 2.0.
In PDF 1.4, support was added for Metadata Streams, using the
Extensible Metadata Platform (XMP) to add XML standards-based
extensible metadata as used in other file formats. PDF 2.0 allows
metadata to be attached to any object in the document, such as
information about embedded illustrations, fonts, and images, as well
as the whole document (attaching to the document catalog), using an
extensible schema.
PDF documents can also contain display settings, including the page
display layout and zoom level in a Viewer Preferences object. Adobe
Reader uses these settings to override the user's default settings
when opening the document. The free Adobe Reader cannot remove these
settings.
Accessibility
===============
PDF files can be created specifically to be accessible to people with
disabilities. PDF file formats in use can include tags, text
equivalents, captions, audio descriptions, and more. Some software can
automatically produce tagged PDFs, but this feature is not always
enabled by default. Leading screen readers, including JAWS,
Window-Eyes, Hal, and Kurzweil 1000 and 3000 can read tagged PDFs.
Moreover, tagged PDFs can be re-flowed and magnified for readers with
visual impairments. Adding tags to older PDFs and those that are
generated from scanned documents can present some challenges.
One of the significant challenges with PDF accessibility is that PDF
documents have three distinct views, which, depending on the
document's creation, can be inconsistent with each other. The three
views are (i) the physical view, (ii) the tags view, and (iii) the
content view. The physical view is displayed and printed (what most
people consider a PDF document). The tags view is what screen readers
and other assistive technologies use to deliver high-quality
navigation and reading experience to users with disabilities. The
content view is based on the physical order of objects within the
PDF's content stream and may be displayed by software that does not
fully support the tags' view, such as the Reflow feature in Adobe's
Reader.
PDF/UA, the International Standard for accessible PDF based on ISO
32000-1 was first published as ISO 14289-1 in 2012 and establishes
normative language for accessible PDF technology.
Multimedia
============
'Rich Media PDF' is a PDF file including interactive content that can
be embedded or linked within the file. It can contain images, audio,
video content, or buttons. For example, if the interactive PDF is a
digital catalog for an E-commerce business, products can be listed on
the PDF pages and can be added with images and links to the website
and buttons to order directly from the document.
Forms
=======
'Interactive Forms' is a mechanism to add forms to the PDF file
format. PDF currently supports two different methods for integrating
data and PDF forms. Both formats today coexist in the PDF
specification:
* AcroForms (also known as Acrobat forms), introduced in the PDF 1.2
format specification and included in all later PDF specifications.
* XML Forms Architecture (XFA) forms, introduced in the PDF 1.5 format
specification. Adobe XFA Forms are not compatible with AcroForms. XFA
was deprecated from PDF with PDF 2.0.
AcroForms were introduced in the PDF 1.2 format. AcroForms permit the
uses of objects ('e.g.' text boxes, Radio buttons, 'etc.') and some
code ('e.g.' JavaScript). Alongside the standard PDF action types,
interactive forms (AcroForms) support submitting, resetting, and
importing data. The "submit" action transmits the names and values of
selected interactive form fields to a specified uniform resource
locator (URL). Interactive form field names and values may be
submitted in any of the following formats, (depending on the settings
of the action's ExportFormat, SubmitPDF, and XFDF flags):
; HTML Form format: HTML 4.01 Specification since PDF 1.5; HTML 2.0
since 1.2
; Forms Data Format (FDF): based on PDF, uses the same syntax and has
essentially the same file structure, but is much simpler than PDF
since the body of an FDF document consists of only one required
object. Forms Data Format is defined in the PDF specification (since
PDF 1.2). The Forms Data Format can be used when submitting form data
to a server, receiving the response, and incorporating it into the
interactive form. It can also be used to export form data to
stand-alone files that can be imported back into the corresponding PDF
interactive form. FDF was originally defined in 1996 as part of ISO
32000-2:2017.
; XML Forms Data Format (XFDF): (external XML Forms Data Format
Specification, Version 2.0; supported since PDF 1.5; it replaced the
"XML" form submission format defined in PDF 1.4) the XML version of
Forms Data Format, but the XFDF implements only a subset of FDF
containing forms and annotations. Some entries in the FDF dictionary
do not have XFDF equivalents - such as the Status, Encoding,
JavaScript, Page's keys, EmbeddedFDFs, Differences, and Target. In
addition, XFDF does not allow the spawning, or addition, of new pages
based on the given data; as can be done when using an FDF file. The
XFDF specification is referenced (but not included) in PDF 1.5
specification (and in later versions). It is described separately in
'XML Forms Data Format Specification'. The PDF 1.4 specification
allowed form submissions in XML format, but this was replaced by
submissions in XFDF format in the PDF 1.5 specification. XFDF conforms
to the XML standard. XFDF can be used in the same way as FDF; e.g.,
form data is submitted to a server, modifications are made, then sent
back and the new form data is imported in an interactive form. It can
also be used to export form data to stand-alone files that can be
imported back into the corresponding PDF interactive form. As of
August 2019, XFDF 3.0 is an ISO/IEC standard under the formal name
'ISO 19444-1:2019 - Document management -- XML Forms Data Format --
Part 1: Use of ISO 32000-2 (XFDF 3.0)'. This standard is a normative
reference of ISO 32000-2.
; PDF
The entire document can be submitted rather than individual fields and
values, as was defined in PDF 1.4.
AcroForms can keep form field values in external stand-alone files
containing key-value pairs. The external files may use Forms Data
Format (FDF) and XML Forms Data Format (XFDF) files. The usage rights
(UR) signatures define rights for import form data files in FDF, XFDF,
and text (CSV/TSV) formats, and export form data files in FDF and XFDF
formats.
In PDF 1.5, Adobe Systems introduced a proprietary format for forms;
Adobe XML Forms Architecture (XFA). Adobe XFA Forms are not compatible
with ISO 32000's AcroForms feature, and most PDF processors do not
handle XFA content. The XFA specification is referenced from ISO
32000-1/PDF 1.7 as an external proprietary specification and was
entirely deprecated from PDF with ISO 32000-2 (PDF 2.0).
Licensing
======================================================================
Anyone may create applications that can read and write PDF files
without having to pay royalties to Adobe Systems; Adobe holds patents
to PDF, but licenses them for royalty-free use in developing software
complying with its PDF specification.
Changes to content
====================
In November 2019, researchers from Ruhr University Bochum and
Hackmanit GmbH published attacks on digitally signed PDFs. They showed
how to change the visible content in a signed PDF without invalidating
the signature in 21 of 22 desktop PDF viewers and 6 of 8 online
validation services by abusing implementation flaws.
At the same conference, they additionally showed how to exfiltrate the
plaintext of encrypted content in PDFs. In 2021, they showed new
so-called 'shadow attacks' on PDFs that abuse the flexibility of
features provided in the specification. An overview of security issues
in PDFs regarding denial of service, information disclosure, data
manipulation, and arbitrary code execution attacks was presented by
Jens Müller.
Malware vulnerability
=======================
Some popular PDF readers have a history of security vulnerabilities
that allows PDF files that have been infected with viruses, Trojans,
and other malware to inflict damage. Such PDF files can have hidden
JavaScript code that might exploit vulnerabilities in a PDF reader,
hidden objects executed when the file that hides them is opened, and,
less commonly, a malicious PDF can launch malware.
PDF attachments carrying viruses were first discovered in 2001. The
virus, named 'OUTLOOK.PDFWorm' or 'Peachy', uses Microsoft Outlook to
send itself as an attached Adobe PDF file. It was activated with Adobe
Acrobat, but not with Acrobat Reader.
Over the years, several vulnerabilities have been discovered in
various versions of Adobe Reader, which prompted the company to issue
security fixes. Vulnerabilities have been discovered in other PDF
readers as well. One aggravating factor is that a PDF reader can be
configured to start automatically if a web page has an embedded PDF
file, providing a vector for attack. If a malicious web page contains
an infected PDF file that takes advantage of a vulnerability in the
PDF reader, the system may be compromised even if the browser is
secure. Some of these vulnerabilities are a result of badly written
PDF readers mishandling JavaScript embedded in the PDF file. Disabling
JavaScript execution in the PDF reader can help mitigate such future
exploits, although it does not protect against exploits in other parts
of the PDF viewing software. Some security experts say that JavaScript
is not essential for a PDF reader and that the security benefit that
comes from disabling JavaScript outweighs any compatibility issues
caused. One way of avoiding PDF file exploits is to have a local or
web service convert files to another format before viewing.
On March 30, 2010, security researcher Didier Stevens reported an
Adobe Reader and Foxit Reader exploit that runs a malicious executable
if the user allows it to launch when asked.
Viewers and editors
=====================
Many PDF viewers are provided free of charge from a variety of
sources. Programs to manipulate and edit PDF files are available,
usually for purchase.
There are many software options for creating PDFs, including the PDF
printing capabilities built into macOS, iOS, and most Linux
distributions. Much document processing software including
LibreOffice, Microsoft Office 2007 (if updated to SP2) and later,
WordPerfect 9, and Scribus can export documents in PDF. There are many
PDF print drivers for Microsoft Windows, the pdfTeX typesetting
system, the DocBook PDF tools, applications developed around
Ghostscript and Adobe Acrobat itself as well as Adobe InDesign, Adobe
FrameMaker, Adobe Illustrator, Adobe Photoshop, that allow a "PDF
printer" to be set up, which when selected sends output to a PDF file
instead of a physical printer. Google's online office suite Google
Docs allows uploading and saving to PDF. Some web apps offer free PDF
editing and annotation tools.
The Free Software Foundation was "developing a free, high-quality and
fully functional set of libraries and programs that implement the PDF
file format and associated technologies to the ISO 32000 standard", as
one of its high priority projects. In 2011, however, the GNU PDF
project was removed from the list of "high priority projects" due to
the maturation of the Poppler library, which has enjoyed wider use in
applications such as Evince with the GNOME desktop environment.
Poppler is based on Xpdf code base. There are also commercial
development libraries available as listed in List of PDF software.
The Apache PDFBox project of the Apache Software Foundation is an open
source Java library, licensed under the Apache License, for working
with PDF documents.
Printing
==========
Raster image processors (RIPs) are used to convert PDF files into a
raster format suitable for imaging onto paper and other media in
printers, digital production presses and prepress in a process known
as rasterization. RIPs capable of processing PDF directly include the
Adobe PDF Print Engine from Adobe Systems and Jaws and the Harlequin
RIP from Global Graphics.
In 1993, the Jaws raster image processor from Global Graphics became
the first shipping prepress RIP that interpreted PDF natively without
conversion to another format. The company released an upgrade to its
Harlequin RIP with the same capability in 1997.
Agfa-Gevaert introduced and shipped Apogee, the first prepress
workflow system based on PDF, in 1997.
Many commercial offset printers have accepted the submission of
press-ready PDF files as a print source, specifically the PDF/X-1a
subset and variations of the same. The submission of press-ready PDF
files is a replacement for the problematic need for receiving
collected native working files.
In 2006, PDF was widely accepted as the standard print job format at
the Open Source Development Labs Printing Summit. It is supported as a
print job format by the Common Unix Printing System and desktop
application projects such as GNOME, KDE, Firefox, Thunderbird,
LibreOffice and OpenOffice have switched to emit print jobs in PDF.
Some desktop printers also support direct PDF printing, which can
interpret PDF data without external help.
Native display model
======================
PDF was selected as the "native" metafile format for macOS (originally
called Mac OS X), replacing the PICT format of the earlier classic Mac
OS. The imaging model of the Quartz graphics layer is based on the
model common to Display PostScript and PDF, leading to the nickname
'Display PDF'. The Preview application can display PDF files, as can
version 2.0 and later of the Safari web browser. System-level support
for PDF allows macOS applications to create PDF documents
automatically, provided they support the OS-standard printing
architecture. The files are then exported in PDF 1.3 format according
to the file header. When taking a screenshot under Mac OS X versions
10.0 through 10.3, the image was also captured as a PDF; later
versions save screen captures as a PNG file, though this behavior can
be set back to PDF if desired.
Annotation
============
Adobe Acrobat is one example of proprietary software that allows the
user to annotate, highlight, and add notes to already created PDF
files. One UNIX application available as free software (under the GNU
General Public License) is PDFedit. The freeware Foxit Reader,
available for Microsoft Windows, macOS and Linux, allows annotating
documents. Tracker Software's PDF-XChange Viewer allows annotations
and markups without restrictions in its freeware alternative. Apple's
macOS's integrated PDF viewer, Preview, does also enable annotations
as does the open-source software Skim, with the latter supporting
interaction with LaTeX, SyncTeX, and PDFSync and integration with
BibDesk reference management software. Freeware Qiqqa can create an
annotation report that summarizes all the annotations and notes one
has made across their library of PDFs. The Text Verification Tool
exports differences in documents as annotations and markups.
There are also web annotation systems that support annotation in pdf
and other document formats. In cases where PDFs are expected to have
all of the functionality of paper documents, ink annotation is
required.
Conversion and Information Extraction
=======================================
PDF's emphasis on preserving the visual appearance of documents across
different software and hardware platforms poses challenges to the
conversion of PDF documents to other file formats and the targeted
extraction of information, such as text, images, tables, bibliographic
information, and document metadata. Numerous tools and source code
libraries support these tasks. Several labeled datasets to test PDF
conversion and information extraction tools exist and have been used
for benchmark evaluations of the tool's performance.
Alternatives
======================================================================
The Open XML Paper Specification is a competing format used both as a
page description language and as the native print spooler format for
Microsoft Windows since Windows Vista.
Mixed Object: Document Content Architecture is a competing format.
MO:DCA-P is a part of Advanced Function Presentation.
See also
======================================================================
* ebook
* Web page
* XSL Formatting Objects
* Page margin
* PDF portfolio
Further reading
======================================================================
;ISO Standards
* PDF 2.0
* PDF 2.0
;Adobe open source standards
*
[
https://opensource.adobe.com/dc-acrobat-sdk-docs/pdfstandards/PDF32000_2008.pdf
PDF 1.7 (ISO 32000-1:2008)]
*
[
https://opensource.adobe.com/dc-acrobat-sdk-docs/pdfstandards/pdfreference1.7old.pdf
PDF 1.7] and
*
[
https://opensource.adobe.com/dc-acrobat-sdk-docs/pdfstandards/pdfreference1.6.pdf
PDF 1.6] () and
*
[
https://opensource.adobe.com/dc-acrobat-sdk-docs/pdfstandards/pdfreference1.5_v6.pdf
PDF 1.5] and
*
[
https://opensource.adobe.com/dc-acrobat-sdk-docs/pdfstandards/pdfreference1.4.pdf
PDF 1.4] () and
*
[
https://opensource.adobe.com/dc-acrobat-sdk-docs/pdfstandards/pdfreference1.3.pdf
PDF 1.3] () and
*
[
https://opensource.adobe.com/dc-acrobat-sdk-docs/pdfstandards/pdfreference1.2.pdf
PDF 1.2]
*
[
https://opensource.adobe.com/dc-acrobat-sdk-docs/pdfstandards/pdfreference1.0.pdf
PDF 1.0] ()
;Conference papers
*
External links
======================================================================
* [
https://pdfa.org/ PDF Association] - The PDF Association is the
industry association for software developers producing or processing
PDF files.
** [
https://pdfa.org/resource/pdf-specification-index/ PDF
Specification Index] at the PDF Association
** [
https://pdfa.org/resource/pdf-cheat-sheets/ PDF Cheat Sheets, 2nd
edition] by the PDF Association (last updated October 29, 2024)
**
[
https://www.pdfa-inc.org/product/iso-32000-2-pdf-2-0-bundle-sponsored-access/
Sponsored free access to the ISO 32000-2 (PDF 2.0) bundle], including
the latest core PDF specification and five ISO standardized extensions
to the core specification
* Format description of
[
https://www.loc.gov/preservation/digital/formats/fdd/fdd000030.shtml
the PDF family],
[
https://www.loc.gov/preservation/digital/formats/fdd/fdd000318.shtml
PDF/A],
[
https://www.loc.gov/preservation/digital/formats/fdd/fdd000124.shtml
PDF/X] from Library of Congress
;Tech notes from Adobe
*
* - Official introductory comparison of PS, EPS vs. PDF.
*
;Articles
*
[
http://preserve.mactech.com/articles/mactech/Vol.15/15.09/PDFIntro/index.html
Portable Document Format: An Introduction for Programmers (1999)] from
MacTech - Introduction to PDF vs. PostScript and PDF internals (up to
v1.3)
* from Planet PDF that describes the paper in which John Warnock
outlined the project that created PDF
;Videos
* [
https://www.youtube.com/watch?v=poc9PVmFzpc Video: Everything you
wanted to know about PDF but was afraid to ask] Recording of a talk
by Leonard Rosenthol (Adobe Systems) at TUG 2007
License
=========
All content on Gopherpedia comes from Wikipedia, and is licensed under CC-BY-SA
License URL:
http://creativecommons.org/licenses/by-sa/3.0/
Original Article:
http://en.wikipedia.org/wiki/PDF
