Require a password to open or modify a file
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 file.
On the Tools menu, click Options, and then click Security.
Do one of the following:
Create a password to open

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.
Create a password to modify

In the Password to modify box, type a password, and then click OK.
In the Reenter password to modify box, type the password again, and then click OK.