Julia Haley recently took me to task for being a hypocrite on the subject of PDFs. In general, PDFs as permanent elements of a web site are a bad idea, especially, as is commonly the case, when used as a lazy form of posting content, such as your annual report or a study.
There are exceptions though. In our everyday lives, we routinely download PDFs of basketball tickets and airline boarding passes — documents that we are almost certainly going to print out because the value is in the printed, as opposed to the online version.
Can you think of examples of PDFs that your audience will actually welcome?