http://www.hotscripts.com/category/p...ipulation/pdf/ has links to 17 different programmes ranging from free to expensive, and including the programme mentioned by late9 above. I've just checked through them all.

lat9
Not sure, about the SetaPdf-Stamper. Looks as though I can ask it to add anything to an existing PDF file, but not sure this would allow me to add something DIFFERENT each time someone bought an eBook. On your site, does it just add that "This eBook was purchased From ..." or something similar (The same each time), or do you use it to add something like "This eBook was purchased BY ..." or something similar (Different each time) ?