protecting PDF ebooks

Hi!
 
This question is about protecting a to-be published PDF ebook.
 
Some of the books we publish change very little over time and have increasing market overseas.
 
We are thinking to offer them as PDF 'ebooks' but are petrified by the thought that a malicious customer (or hundreds) might post the PDF on a public forum (especially our forthcoming book in Chinese) in China.
 
It seems that Acrobat 8.0 has lots of security features (which we don't yet fully understand), but if we start sellling thousands of PDF books is there going to be a 'workflow' solution to distributing different passwords to each customer when they order an ebook thru our online store (hosted by eSellerate.net)?
 
Or is there a better model for protecting the sold document from being passed around and/or posted on the web?
 
BTW: we would have done this long ago, were it not for the story we often heard(true or not, it was enough to scare us off) that breaching Acrobat security using non-Adobe hacking tools was a trivial exercise.
 
Thank you for your time on this.