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
I think you have to use Adobe Reader to view Adobe DRM protected PDF files. Because the different DRM technologies will generate different DRM PDF files, they are incompatible between each other. For example, Adobe Reader can't view VeryPDF DRM PDF files, VeryPDF Reader can't read Adobe DRM PDF files, and so on.
So, the best choice is use Adobe Reader to read Adobe DRM PDF files.
Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted.
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.