If you want to remind everyone that a workbook should only be changed if necessary, you can recommend that the workbook be opened as read-only.
Note: Prompting to open as read-only will not prevent changes from being made to your workbook. For stronger protection, set read-only status by locking the workbook with the Finder, or require a password to save changes.
Open the workbook that you want to protect.
On the Review tab, under Protection, click Passwords.
Select the Read-only recommended check box, and click OK.
Close the workbook.
The next time the workbook is opened, a message will ask if you want to open as read-only.