DRM and pdf

I'm working for an Italian company and we'd like to start and sell
eBooks on our e-learning platform.
So we need a system, easy to integrate with our online store, in such a
manner that autorized users (whose who buyed our pdf e-book online)
might see the book under some conditions (for example only on one
I've read a lot about third party DRM and finally I've found FileOpen
system; do you have any experience with DRM or FileOpen system? I'm not
sure if it's what we really need...

Could you suggest me something about other products?

Thank you very much,