Add a password to a PDF
Open the PDF, then click the File menu followed by Properties
Click the Security tab, then click the drop-down menu beside Security Method, and select Password Security from the list of resulting list of options.
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.
Click OK, and then confirm the password.
Click OK again and notice that you must save the PDF before your changes take effect.
Click OK on the Properties dialog and save your work.