Password protect a document
You can help prevent unauthorized users from opening a document or from modifying a document even if they have permission to open it.
When you create a password for a document, write down the password and keep it in a secure place. If you lose the password, you can't open or gain access to the password-protected document.
Passwords in Excel for Mac and Word for Mac have a 15-character limit. You cannot open a workbook or document that has been password protected in the Windows-based version of Excel or Word if the password is longer than fifteen characters. If you want to open the file in Office for Mac, ask the Windows-based author to adjust the password length.
Word doesn't encrypt the data that is hidden in a document. Given sufficient time and knowledge, a user can modify data in any document that he or she has access to. To help prevent modification of data, and to help protect confidential information, limit access to any Word documents that contain such information by storing documents in locations available only to authorized users.
Do any of the following:
Require a password to open a document
Require that others enter a password to modify a document
Protect a document during review
Change a document password