Assign a Password to Open from the Excel Info Screen
Type the password in the Reenter Password text box exactly as you typed it into the Password text box in the Encrypt Document dialog box and then select OK.
Note that if you don’t replicate the password exactly, Excel displays an alert dialog box indicating that the confirmation password is not identical. After you click OK to close this alert dialog box, you’re returned to the Confirm Password dialog box.
After successfully replicating the password, Excel closes the Confirm Password dialog box and returns you to the Info screen, where A password is required to open this workbook status message now appears under the Protect Workbook heading.