When you create a password, write it down and keep it in a secure place. If you lose the password, you cannot open or gain access to the password-protected document.
- Open the document.
- On the File menu, click Save As.
- On the Tools menu in the Save As dialog box, click General Options.
- In the Password to open box, type a password, and then click OK.
- In the Reenter password to open box, type the password again, and then click OK.
- Click Save.
(from Tom Hall)