With the file open in Pages, Numbers, or Keynote, select File and then Set Password to choose and apply your password.

Adobe made a critical change to the algorithm used to password-protect PDF documents in Acrobat 9, making it much easier to recover a password and raising concern over the safety of documents, ...

Changes to the algorithm used to password-protect PDF documents in Acrobat 9 make it much easier to recover a password and raise concern over the safety of documents Adobe made a critical change to ...