When encrypting a PDF in Adobe Acrobat Pro XI, there's a message to the effect that some third-party programs which read PDFs may ignore the encryption password and open the PDF for viewing. Is Adobe PDF encryption really no more than a sign on a door that reads
This door is locked. The key is in the lock. But don't use it.
All Adobe products enforce the restrictions set by the Permissions Password. However, not all third-party products fully support and respect these settings. Recipients using such third-party products might be able to bypass some of the restrictions you have set.