The following has worked for me. (I have used uppercase for the main points.)
- Before you do anything, consider copying the file to a new name, so you can keep the
original unprotected one. (You can delete it later if you want to.)
- Open your pdf in Preview by right-click, select Open With, and choose Preview
- In File menu of Preview, click on EDIT PERMISSIONS
- On the dialog box, click on the square for Require Password to Open Document
- Type and verify the password you are giving the file
- Where you see Permissions, UNCLICK EVERYTHING
- Type and verify the Owner Password, which can be same or different as the other one.
- Click on Apply
- Close the file (you don’t even need to Save)
- When you want to use the file again, you can open it with Preview (though you don’t have
to) especially if you want to change permissions again.
Hope this helps.