I’ve had the need to append pages to locked PDFs. A great tool for unlocking locked PDFs is a service called pdfclean for OS X, an installer of the command line utility included with mupdf’s MuTool clean. I successfully installed and used the service under Mountain Lion and found it effective. Thanks, be3n!

** UPDATE – November 3, 2021 **

For macOS Sonoma and later, be3n’s tool now works as an Automator app, just drop a locked .pdf on it. There may be new issues with this app under Sonoma and I am checking with be3n on a potential solution. Also worth noting, MuTool can strip encryption from a pdf if you provide it the document’s password.