If you are asking how to create them though. I would suggest you download cutepdf. Cutepdf.com it adds itself to the list of printers on your system and let's you create pdf files out of anything you could possibly print!!
Does anyone know how to make these work? I bought an e-book as a pdf four years ago, and now it doesn't work. I didn't think these files were supposed to expire. I'm open to suggestions.