Had an ugly piece of spyware sent to me the other day. It's a PDF that every time you turn a page reports back to a central tracking system, recording your every move, time spent, physical location, details of your mail client and so on. What's worse is that it doesn't even warn you that it's doing it.
Read Notify are the people providing this ignoble service. Certainly I can understand people's desire to track information concerning out bound documents, but it is the lack of warning and the depth of information reported that raises my hackles.
You can defeat the little tracker on PDF if you have a full copy of Acrobat. Just export all pages as Encapsulated Postscript (will export one file per page) and then create a new PDF, importing all of the previously exported EPS files.