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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, colour, and images.
Free readers for many platforms are available for download from the Adobe website , and there are several free open source readers, including Xpdf
for POSIX-like systems with the X
Window System; GPdf
, a derivative of Xpdf for GNOME; GSPdf and ViewPDF , for GNUstep; and front-ends for many platforms to Ghostscript, q.v.
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 is a computer language -- or more accurately 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 PS interpreter. However the entire PS 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 the PS code outputs
are collected and tokenized, any files, graphics or fonts the document references
are also collected, and then everything is compressed into a single file.
Advantages
As a document format, PDF has several advantages over PostScript. One is that a document resides in a single file, whereas the
same document in PostScript may span multiple files (graphics, etc.) and probably occupies more space. In addition, PDF contains
already-interpreted results of the PostScript source code, so it is less computation-intensive and faster to open. Finally, 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.
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. Additionally, there were competing formats. 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.
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, is based on both the Display PostScript standard and PDF, and is sometimes referred to as
DisplayPDF. 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 is not intrinsically inaccessible 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 addding tags to existing or "legacy" PDFs. Moreover,
that process itself is inaccessible. Nonetheless, well-made PDFs can be a valid choice as long-term accessible documents.
Comparison between PDF and HTML
PDF and HTML are not equivalent technologies, but are both commonly found on the
Web.
HTML is a method for describing the content of a webpage in a manner that is open
to interpretation by the browser which renders it on the user's screen. This permits content to be rendered to suit the viewer
rather than the content provider. PDF on the other hand is strictly concerned with describing the content of a document such that
the original intention of its author is fully preserved.
Since many content providers do not like the fluid nature of HTML rendering, PDF has become widespread to force a particular
layout. With HTML the same can be achieved by using a raster graphics image to present text, but then the text can not be copied
as such, nor can a subtext be searched within it. Use of images also leads to larger file sizes. (Sometimes the same is done in a
PDF file, and the same disadvantages apply.)
A typical example of the differences this leads to for the viewer is with zooming:
- Enlarging a PDF document magnifies the text but preserves the original layout and spacing; a practical limit on zooming
follows from the requirement to keep a text column within the width of the screen (otherwise horizontal scrolling would be needed during and after reading each line, which would be very cumbersome).
- With HTML a larger font size is used and lines re-wrap accordingly to fit the browser window. Note, though, that tagged PDFs
can be reflowed quite well, keeping columns of text within the viewable area.
Searching for a text in a collection of files
When searching for a text in a collection of files, it may or may not be possible to search PDF files, depending on the search
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, Google searches PDF
files. During the search Google automatically converts PDF files to HTML and offers the option "View as HTML" (this conversion
does not include images). Mac OS X, having PDF as a core part of the OS, fully supports searching PDF files with the Preview application, used to view PDF files.
Layers
A PDF file for e.g. a map is often a combination of a vector graphics layer
and text, and possibly a raster graphics layer. E.g., the general
reference map of the US [1] uses:
- 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)
- text stored as such - ditto, and also one can copy the text
- raster graphics for showing mountain relief - on zooming in, this consists of enlarged pixels (the blue of the sea and lakes
is "filled" neatly to the vector graphics coast line, hence not part of this raster graphics layer)
An example of a PDF map without raster graphics is [2] . In the map [3] the blue of the sea is
not "filled" neatly to the vector graphics coast line, but just part of the raster graphics layer, giving a cruder result
(noticeable when highly zoomed in).
Tools
- Doc2pdf - email robot
- gnujpdf
- Go2PDF
- GNOME PDF Viewer
- iText - Java
- libharu - C/C++
- OpenOffice.org can transform many types of documents into PDF
documents
- OttoPDF
- Panda library
- PDFBox - Java
- PDFCreator - a GPL PDF printer driver for Windows
- PDF/X-3
Inspector
- PJX
- sharpPDF - C#
- PDF Viewer for X
See also
External links
This article has been created using material from FOLDOC.
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