1.) Open the PDF and choose Tools > Protect > Encrypt > Encrypt with Password.
2.) If you receive a prompt, click Yes to change the security.
3.) Select Require A Password To Open The Document, then type the password in the corresponding field. For each keystroke, the password strength meter evaluates your password and indicates the password strength.
4.) Select an Acrobat version from the Compatibility drop-down menu. Choose a version equal to or lower than the recipient’s version of Acrobat or Reader.
The Compatibility option you choose determines the type of encryption used. It is important to choose a version compatible with the recipient’s version of Acrobat or Reader. Credit Technologies’ version of Acrobat or Reader is : Acrobat 7.0 or Later .
5.) Select an encryption option: Encrypt All Document Contents — Ecrypts the document and the document metadata. If this option is selected, search engines cannot access the document metadata.
6.) Click OK. At the prompt to confirm the password, retype the appropiate password in the box and click OK.