What is the best way to protect e-book?

I am a writer and want to sell my ebooks on the web. But recently there are many bad buyers who remove the protection, modify the contents, and share them on the web.

My related question: Is it true that strongly-password-protected PDF can be cracked?

I want something that can prevent someone from removing the security protection (permission to print, edit, making comments, etc) in my ebooks. I also want to assign a unique identifier for each buyer so I can trace who shares the book (when I get the copy on the web).

How to accomplish my scenario with minimal hassle?

My ebooks are sold in PDF format.

I have found several discussions, and most agree that using Guard Book Pro seems to be the best way (I have no connection with this company).

http://bookguardpro.com/protect-pdf-blog/139/best-way-to-protect-my-pdf-ebook/

Others have opined that maybe it is not the worst thing in the work if your book gets passed around, as that is free publicity. My own two cents would say that if it were a novel especially, this may be a great approach. Novelists usually get rich on their SECOND novel, only because the first one made them famous.

http://www.linkedin.com/groups/Can-anybody-suggest-better-way-1515307.S.41251106

Lastly, if you want to keep if from spreading, I liked this last idea a lot too: Add the person’s Paypal name and address to every book you send out. That won’t stop everyone either, but it will stop the vast majority of ordinary users. Would you want that info to be passed around?

http://www.blackhatworld.com/blackhat-seo/black-hat-seo-tools/250227-want-protect-pdf-ebook.html

There is still no perfect way, but these will minimize your exposure...IF that is ultimately what you really want.