How to Protect Your Excel Workbook when Saving the File
(Optional) If you want to assign a password to open the file, type the password (up to 255 characters maximum) in the Password to Open text box.
As you type the password, Excel masks the actual characters you type by rendering them as dots in the text box.
If you decide to assign a password for opening and modifying the workbook at the same time, proceed to Step 3. Otherwise, skip to Step 4.
When entering the password for modifying the workbook, you want to assign a password that’s different from the one you just assigned for opening the file (if you did assign a password for opening the file in this step).