You can make Excel always open old spreadsheets in Protected View. Enable or disable Protected View using in-built setting, Group Policy, Registry.

With the Excel file open select File in the top-left corner of the screen, followed by Info. Select Protect Workbook, then choose Encrypt with Password.

There are several options available to you to protect your Excel file with a password. Remember that once you protect the Excel file, you cannot open it without entering the password.

At the file level, you can password protect an Excel workbook in two ways: You can determine who can get in and who can save changes.

However, if you open a protected Excel file in the browser edition, you can see hidden formulas, but you can’t modify them. For most sheets, hiding formulas is overkill but easily implemented.