Free readers for many platforms are available for download from the Adobe website; and a free open source reader is available from Xpdf. Xpdf , and GNOME Pdf are among several independent PDF viewers and interfacing libraries that have been developed for POSIX-like systems (and ported to other platforms). The KDE browser and file manager Konqueror are also PDF viewers.
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 computer language -- or more accurately a page description language -- that is run in an interpreter to generate an image. This complex process requires a fair amount of resources in order to work.
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 while 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's Acrobat 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 closely based on the Display PostScript standard, and is thus highly compatible with PDF. Because of the OS support, all OS X applications can create PDF documents automatically as long as they support the Print command.
Comparison between reading a PDF file and a HTML file
PDF and HTML are not equivalent technologies, but are both commonly found on the web.
HTML vs. PDF
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.
Difference
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. (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.
Searching for a text in a collection of files
When searching for a text in a collection of files, depending on the search program, it may or may not be possible to search PDF-files. This is because the text is stored in coded form, and a program searching for a text should interpret the code and not just search the code itself. Search programs that do not work include that of Windows XP and Agent Ransack. However, for searching the web, Google also searches PDF-files. Mac OS X, having PDF as a core part of the OS, implements searches fully within 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)
* 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)
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