If you are using Office 2011 for Mac, here is what you want.
Require a password to open a document
Open the document that you want to help protect.
On the Word menu, click Preferences.
Under Personal Settings, click Security .
In the Password to open box, type a password, and then click OK.
In the Confirm Password dialog box, type the password again, and then click OK.
Click Save .
Tip To remove a password, select all contents in the Password to open box, and then press DELETE.