The Portable Document Format (PDF) is the file format created by Adobe Systems, in 1993, for document exchange. PDF is used for representing two-dimensional documents in a device-independent and display resolution-independent fixed-layout document format. Each PDF file encapsulates a complete description of a 2-D document (and, with Acrobat 3-D, embedded 3-D documents) that includes the text, fonts, images, and 2-D vector graphics that compose the document.
PDF is an open standard, and is now being prepared for submission as an ISO standard.

History
Anyone may create applications that 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.
The PDF combines three technologies:

A sub-set of the PostScript page description programming language, 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. Technology
PostScript is a page description language run in an interpreter to generate an image, a process requiring many resources. PDF is a file format, not a programming language, i.e. flow control commands such as if and loop are removed, while graphics commands such as lineto remain.
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 to which the document refers also are collected; then, everything is compressed to a single file. Therefore, the entire PostScript world (fonts, layout, measurements) remains intact.
As a document format, PDF has several advantages over PostScript:

PDF contains already tokenized and interpreted results of the PostScript source code, for direct correspondence between changes to items in the PDF page description and changes to the resulting page appearance.
PDF (from version 1.4) supports true graphic transparency, PostScript does not.
PostScript is an imperative programming language (with an implicit global state), so instructions accompanying the description of one page can affect the appearance of any following page. Therefore, all preceding pages must be processed in order to determine the correct appearance of a given page; each page in a PDF document is unaffected by the others. Accessibility
PDFs may be encrypted so that a password is needed to view or edit the contents. The PDF Reference defines both 40-bit and 128-bit encryption, both making use of a complex system of RC4 and MD5. The PDF Reference also defines ways in which third parties can define their own encryption systems for use in PDF.
PDF files may also contain embedded DRM restrictions that provide further controls that limit copying, editing or printing. The restrictions on copying, editing, or printing depend on the reader software to obey them, so the security they provide is limited. Printable documents especially might be saved instead as bitmaps and subject to OCR.
The PDF Reference has technical details or see [1] for an end-user overview. Like HTML files, PDF files may submit information to a web server. This could be used to track the IP address of the client PC, a process known as phoning home.
Through their LiveCycle Policy Server product, Adobe provides a method to set security policies on specific documents. This can include requiring a user to authenticate and limiting the time frame a document can be accessed or amount of time a document can be opened while offline. Once a PDF document is tied to a policy server and a specific policy, that policy can be changed or revoked by the owner. This controls documents that are otherwise "in the wild." Each document open and close event can also be tracked by the policy server. Policy servers can be set up privately or Adobe offers a public service through Adobe Online Services.

Usage restrictions and monitoring
Proper subsets of PDF have been, or are being, standardized under ISO for several constituencies:
A PDF/H variant (PDF for Healthcare) is being developed. However, it may consist more in a set of "best practices" than in a specific format or subset.

PDF/X for the printing and graphic arts as ISO 15930 (working in ISO TC130)
PDF/A for archiving in corporate/government/library/etc environments as ISO 19005 (work done in ISO TC171)
PDF/E for exchange of engineering drawings (work done in ISO TC171)
PDF/UA for universally accessible PDF files Subsets
According to a 7 December 2006 Government Computer News blog, Joab Jackson writes that Adobe is exploring an XML-based next-generation PDF codenamed Mars: http://www.gcn.com/blogs/tech/42740.html
Adobe has published information about the Mars file format at http://www.adobe.com/go/mars and at http://labs.adobe.com/wiki/index.php/Mars.

Mars
A PDF file is often a combination of vector graphics, text, and raster graphics. The basic types of content in a PDF are:
In later PDF revisions, a PDF document can also support links (inside document or web page), forms, JavaScript (initially available as plugin for Acrobat 3.0), or any other types of embedded contents that can be handled using plug-ins.
PDF 1.6 supports interactive 3D documents embedded in the PDF.
Two PDF files which look similar on a computer screen may be of very different sizes. For example, a high resolution raster image takes more space than a low resolution one. Typically higher resolution is needed for printing documents than for displaying them on screen. Other things that may increase the size of a file is embedding full fonts, especially for Asiatic scripts, and storing text as graphics.

text stored as such
vector graphics for illustrations and designs that consist of shapes and lines
raster graphics for photographs and other types of image Content
There are fourteen typefaces that have a special significance to PDF documents: Times Roman (in standard, italic, bold, and bold oblique), Courier (in standard, oblique, bold and bold oblique), Helvetica (in standard, oblique, bold and bold oblique), Symbol and Zapf Dingbats. These should always be present (actually present or a close substitute) and so need not be embedded in a PDF. [2] PDF viewers must know about the metrics of these fonts. Other fonts may be substituted if they are not embedded in a PDF.

Base 14 Fonts

Implementations

List of PDF software
Scalable Vector Graphics
XML Paper Specification
XSL Formatting Objects