Did you know that PDF's can be managed by iTunes. Doug Adams pointed out that this was first mentioned on MacOSXHints.
Apple has clearly added pdf support to iTunes in order to manage album artwork/liner notes. The only known music store files with associated PDF's are U2's "Complete" & "How to Dismantle an Atomic Bomb."
You can even create a PDF Smart Playlist. Doug created a Live Updating Smart Playlist that snags all the PDFs he's added to the Library.