The acronym PDF stands for Protected Document Format. In the early days of the internet, the ability to copy documents was identified as a major threat, and Adobe produced the PDF format as a reader/protection format for documents. It has since become standard use on the net, and while there have been some criticisms of the amount of protect provided, it's generally accepted. Plagiarism and theft of copyright materials has become a serious problem online, and it has to be said that if the PDF format hasn't stamped it out, it has at least made it much more difficult.
Examples of PDF: