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.

If you have sensitive data you'd like to keep from prying eyes in a Microsoft Excel file, the easiest way to protect it is with Excel's built-in password tools.

If you cannot open an Excel file and see Excel cannot open the file because the file format or extension is not valid error, see this post.

You can add or remove a password from your Excel spreadsheets through the File tab. You'll do it slightly differently on a Mac and PC.