The standard security provided by Arobat® PDF consists of two different methods and two different passwords, “user password” and “owner password”. A PDF document may be protected by password to open (‘user’ password) and the document may also specify operations that should be restricted even when the document is decrypted: printing; copying text and graphics out of the document; modifying the document; and adding or modifying text notes and AcroForm fields (using ‘owner’ password). More about permission setting.
If user password (also known as password to open) is either not set or known, it is possible to remove permission password instantly. Unfortunately, if a user password is unknown, there is no easy way to recover the password to a PDF document. Universal recovery methods, such as Brute Force Attack and Dictionary Attack, must be used. Unfortunately, due to a nature of an encryption method used in PDF files, the password search is very slow. The brute force speed is usually under one thousand passwords per second even on a top-speed modern PC. It means that long password recovery takes too long time.