There’s so much to PDF it is difficult to define precisely. Following is the official definition from the PDF Reference 1.6 Chapter 2:
“THE ADOBE PORTABLE DOCUMENT FORMAT (PDF) is a file format for representing documents in a manner independent of the application software, hardware, and operating system used to create them and of the output device on which they are to be displayed or printed.”
The key to understanding PDF is really to think of it as electronic paper. It is designed for device independent and resolution independent viewing and printing of page based documents. While physical paper is relatively simple, electronic paper is far more complex. Maintaining independence from the viewing platforms requires adding a significant amount of extra information to the file.
PDF is based on the Postscript page description language also developed by Adobe Systems.