The standard security provided by PDF consists of two different methods and two different passwords, 'user password' and 'owner password'.

A PDF document may be protected by password for opening ('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).

Limitation:
Please note that PDF Password Remover doesn't work with documents which have 'user' password (preventing the files from being opened), if both user and owner passwords are unknown the PDF Password Remover will fail.