 Portable Document Format (PDF)
is a file format developed by Adobe Systems for representing documents
in a manner that is independent of the original application software,
hardware, and operating system used to create those documents. A PDF
file can describe documents containing any combination of text,
graphics, and images in a device independent and resolution independent
format. These documents can be one page or thousands of pages, very
simple or extremely complex with a rich use of fonts, graphics, color,
and images. PDF is an open standard, and anyone may write applications
that can read or write PDFs royalty-free.
In addition to
encapsulating text and graphics, PDF files are most appropriate for
encoding the exact look of a document in a device-independent way. In
contrast, markup languages such as HTML defer many display decisions to
a rendering device such as a browser, and will not look the same on
different computers. Free
readers for many platforms are available: the free Adobe Reader by
Adobe Systems, the free Foxit Reader , and several free open source
readers, including Xpdf for POSIX-like systems with the X Window
System; KPDF, a viewer based on Xpdf for KDE; GPdf, a derivative of Xpdf
for GNOME; Evince, a document viewer for GNOME (fork of GPdf) that can
view PDF-files; GSPdf and ViewPDF , for GNUstep; and front-ends for
many platforms to Ghostscript.
Proper subsets of PDF, collectively called PDF/X, have been standardized by ISO.
Technology
PDF is primarily the combination of three technologies:
- a cut-down form of PostScript for generating the layout and graphics,
- a font-embedding/replacement system to allow fonts to travel with the documents, and
- a structured storage system to bundle these elements into a single file, with data compression where appropriate.
PostScript
PostScript is a computer
language — more precisely, a page description language — that is run in
an interpreter to generate an image. This process requires a fair
amount of resources. PDF
is a subset of those PostScript language elements that define the
graphics, and only requires a very simple interpreter. For instance,
flow control commands like if and loop are removed, while graphics commands such as lineto remain.
That means
that the process of turning PDF back into a graphic is a matter of
simply reading the description, rather than running a program in the
PostScript interpreter. However, the entire PostScript world in terms
of fonts, layout and measurement remains intact.
Often, the
PostScript-like PDF code is generated from a source PostScript file.
The graphics commands that are output by the PostScript code are
collected and tokenized; any files, graphics or fonts the document
references are also collected; and finally everything is compressed
into a single file.
As a document format, PDF has several advantages over PostScript:
- A PDF document
resides in a single file, whereas the same document in PostScript may
span multiple files (graphics, etc.) and probably occupies more space.
- PDF contains already-interpreted results of the PostScript
source code, so it is less computation-intensive and faster to open,
and there is a more direct correspondence between changes to items in
the PDF page description and changes to the resulting appearance of the
page.
- PDF (starting from version 1.4) supports true object transparency while PostScript does not.
- If displayed with Adobe Reader, a font-substitution strategy
ensures the document will be readable even if the end-user does not
have the "proper" fonts installed. PDF also allows font embedding to
ensure that the "proper" fonts are displayed. While this is possible
with PostScript, such files cannot normally be distributed freely
because of font licensing agreements.
History
When PDF first came out
in the early 1990s, it was slow to catch on. At the time, not only did
the only PDF creation tools of the time (Acrobat) cost money, but so
did the software to view and print PDF files. Early versions of the PDF
format had no support for external hyperlinks, reducing its usefulness
on the web. Additionally, there were competing formats such as Envoy,
Common Ground Digital Paper, DjVu and even Adobe's own PostScript file
format (.ps). Adobe started distributing the Acrobat Reader program at
no cost, and continued to support PDF through its slow multi-year
ramp-up. Competing formats eventually died out, and PDF became a
well-accepted standard. In
2005 Microsoft presented a competing format referenced by the code name
"Metro". It is developed together with Global Graphics. Metro is based
on XML, but requires a license. Metro is scheduled to be included in
the next version of Microsoft Windows Vista.
Macintosh
PDF was selected as the
"native" metafile format for Mac OS X, replacing the PICT format of the
earlier Mac OS. Mac OS X's imaging model, Quartz 2D, is based on both
the Display PostScript standard and PDF, and is sometimes referred to
as Display PDF. Due to OS support, all OS X applications can create PDF
documents automatically as long as they support the Print command.
PDF and accessibility
PDF can be accessible to
people with disabilities. Current PDF file formats can include tags
(essentially XML), text equivalents, captions and audio descriptions,
and other accessibility features. Some software, such as Adobe
InDesign, can output tagged PDFs automatically. Leading screen readers,
including Jaws, Window-Eyes, and Hal, can read tagged PDFs; current
versions of the Acrobat and Acrobat Reader programs can also read PDFs
out loud. Moreover, tagged PDFs can be reflowed and zoomed for
low-vision readers. However,
many problems remain, not least of which is the difficulty in adding
tags to existing or "legacy" PDFs; for example, if PDFs are generated
from scanned documents, accessibility tags and reflowing are
unavailable and must be created either by hand or using OCR techniques.
Also, these processes themselves are often inacessible to the people
who would benefit from them. Nonetheless, well-made PDFs can be a valid
choice as long-term accessible documents. (Work is being done on a PDF
variant based on PDF 1.4. The PDF/A or PDF-Archive is specifically
scaled down for archival purposes.)
Microsoft
Word documents can be converted into accessible PDFs, but only if the
Word document is written with accessibility in mind - for example,
using styles, correct paragraph mark-up and "alt" (alternative) text
for images, and so on.
PDF on the Web
Because HTML/XHTML
rendering across web browsers has historically been inconsistent and
sometimes unpredictable, PDF use online is becoming increasingly
common. This is particularly true for order forms, catalogues,
brochures, and other documents which are primarily formatted for
printing. The ubiquity of the Adobe Reader web browser plugin, however,
has inspired some (mostly corporate) web authors to publish a wider
variety of information as PDF. This trend is compounded by the simple
operation and wide corporate availability of WYSIWYG PDF authoring
tools. While the end user experience of an XHTML document can vary
significantly depending on browser, platform, and screen resolution, a
PDF file can be reasonably expected to look exactly the same to every
viewer. Critics
of this practice cite several reasons for avoiding it. The major one is
that the inflexibility of PDF rendering makes it difficult to read on
screen: it does not adapt to the window size nor the reader's preferred
font size and font family, as classic XHTML web page does.
Accessibility, particularly by the blind or sight-impaired is a common
issue . PDF files tend to be significantly larger than XHTML/SVG files
presenting the same information, making it difficult or impossible for
users with low-bandwidth connections to view them. Adobe Acrobat
Reader, the de facto standard PDF viewer, has historically been slow to
start and caused browser instability, particularly when run alongside
other browser plugins (though the release of Adobe Reader 7 addressed
many of these concerns). Acrobat reader is also unavailable in current
versions on many alternative operating systems and is distributed under
a proprietary license unacceptable to some users.
Currently,
only one major web browser, Safari, natively supports PDF. Users of
other clients must run a separate application to access these documents
online.
Searching for a text in a collection of files
Adobe Acrobat Reader 6.0 and above allow searching a collection of PDF files. Using
a search program to search for a text in a collection of files of
different types, it may or may not be possible to also search PDF
files, depending on the program. This is because the text is stored in
coded form, and a program searching for some text must interpret the
code and search the result, not just search the code. Search
programs that do not work include that of Windows XP and Agent Ransack.
However, for searching the Web, some search engines, such as Google and
Yahoo!, include PDF files in searches. The option to view the PDF in
HTML format is also commonly offered (this conversion does not include
images). Mac OS X,
having PDF as a core element of the operating system, fully supports
searching PDF files with the Preview application, used to view PDF
files. The Spotlight feature in Mac OS X v10.4 extends this ability
across the whole operating system, allowing information in PDF files
(as well as almost all others) to be found from a single search box.
On the
Windows platform, text in PDF files can be searched using Google
Desktop Search and also Windows Desktop Search when installed with an
appropriate iFilter available from Adobe.
Types of content
A PDF file for e.g. a
map is often a combination of vector graphics layer, text, and raster
graphics. For example, the general reference map of the US uses:
- text stored as such — scalable, and also one can copy the text
- vector graphics for coastlines, lakes, rivers, highways,
markings of cities, and Interstate highway symbols — on zooming in, the
curves remain sharp, they do not appear as consisting of enlarged
pixels (i.e. rectangles of pixels)
- raster graphics for showing mountain relief — on zooming in, this consists of enlarged pixels
To appreciate the
difference between vector and raster graphics, compare the general
reference map to the CIA World Factbook's
European map. If you zoom in on the former, you'll see that the
(vector) coastline is nicely fitted to the (raster) mountain relief. If
you zoom in on the latter, in contrast, the blue of the sea is not nicely fitted to the vector graphics coast line, and the land-sea boundary is much cruder. Some PDFs have no raster graphics at all. For example, see the Factbook's
map of the Arctic.
Tools exist,
such as pdfimages (bundled with Xpdf) to extract the raster images from
a PDF file. This can be extremely useful if the PDF is simply a
collection of scanned pages.
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