Portable Document Format (PDF) is an open standard for document exchange.
This file format created by Adobe Systems in 1993 is used for representing documents in a manner independent of application software, hardware, and operating systems. Each PDF file encapsulates a complete description of a fixed-layout flat document, including the text, fonts, graphics, and other information needed to display it.
Source: http://en.wikipedia.org/wiki/Portable_Document_Format