Require a password to open or modify a workbook
You can help prevent unauthorized users from opening or modifying a workbook file, even if they have permission to open it.
When you create a password for a workbook, 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 workbook.
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.
Do any of the following:
Require a password to open a workbook
Require a password to modify a workbook