Adobe makes two versions of their PDF program: standard and professional. It has been awhile, but I think the pro version has additional secuirty.
Unfortunately, there is a lot of confusion around PDF and ebooks when it comes to security. In short--it isn't going to happen without a lot of money. Adobe makes a program specifically for providing PDF ebooks over the Internet. The program cost a few thousand. There are vendors who will put your ebook up for a pricey fee.
If you're thinking that you can lock the PDF and sell the registeration code to unlock the book you might first want to Google for "unlock PDF". There are several very inexpensive programs that will do just that.
If you were thinking of selling on Amazon, then you might want to do some investigation. I've heard that they will set up PDF files for secure download. They use the more advanced program so the PDF file is available to both computers and PDAs while being secure.
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