PDF has been around for a long time, in fact, the first embryonic signs of PDF started to rear their heads in the spring of 1991, when John Warnock penned and released his Camelot whitepaper.
But it has been around for so long, that most people don't actually know what it stands for -- "Portable Document File," "Portable Designer Folder," "Purple Dollar Fish" or, *ahem* something else entirely.
Yes, that's right, I often still get asked, "What is a PDF? What does PDF stand for?" When I do, I first send people over to Duff Johnson's article in the Enterprise and Government Learning Center -- What is PDF? -- and then to Wikipedia, which sports a comprehensive description of the file format.
OK, so you want to stamp your document. Maybe you need to give reviewers some advice about the document's status or sensitivity. This tip from author Ted Padova demonstrates how to add stamps with the Stamp Tool along with related comments.