DRM solutions can mean a forwarded file is useless. Adobe's DRM solution is part of Adobe Experience Manager, and price, by negotiation, is reputed to be 5-6 figures. But here's something else to think about. One of the beauties of PDF is that anyone can open it - in their browser, email, webmail, on their phone or tablet, even TV. The moment you add DRM *all* of that is gone. The user has to know how to save the file and get into the app which handles the DRM, which may not be available on their platform. You are giving a major inconvenience. Bear in mind too that anything on a screen can be captured and resold in minutes.