What is PDF Encryption?
TL;DR: PDF encryption protects documents by scrambling their contents so only authorized users with the correct password can access them. PDFs support two encryption levels: 128-bit AES and 256-bit AES, with the latter being effectively unbreakable with current technology.
PDFs have two types of passwords: an open password (required to view the document) and a permissions password (controls what actions are allowed — printing, copying, editing). Both use symmetric key encryption.