DRM and pdf

Hi!
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
machine).
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,
Davide.

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