How to Protect Your Excel Workbook when Saving the File
Click the Tools button in the Save As dialog box and then choose General Options from its drop-down menu.
Doing this opens the General Options dialog box, where you can enter a password to open and/or a password to modify in the File Sharing section. Your password can be as long as 255 characters, consisting of a combination of letters and numbers with spaces.
When adding letters to your passwords, keep in mind that these passwords are case-sensitive. This means that opensesame and OpenSesame are not the same password because of the different use of upper- and lowercase letters.