Word 2007: Password Protect Your Document File

1. Click the Microsoft Office Button, and then click Save As.
2. Click Tools, and then click General Options.
3. You can type a password in Password to open or Password to modify.
4. Click OK.
5. When prompted, retype your passwords to confirm them, and click OK.
6. Click Save.

ref:- http://www.technipages.com/word-2007-password-protect-your-document-file.html