I would like to know if the Preview application (last release) is able to open PDF files that are DRM protected, for example by Adobe DRM system.
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
Preview can display encrypted PDF files (if you know the password), if those files are encrypted according to the PDF standard.
There are other types of third-party DRM that can be applied to PDFs, and Preview cannot read those files. Adobe also has their own non-standard "Digital Editions" DRM, which Preview cannot read.