OSX Preview: is able to display DRM protected PDF files? (mainly concerned about Adobe DRM)
Preview.app cannot view DRM protected PDFs, the only way to view these files is to change the Default Program for PDFs to Acrobat.

Personally, I would not advocate this due to the unsecure nature of Adobe Reader on OSX (and Windows) and would go through the onetime process to remove the DRM restrictions on each PDF