Excel (Mac) - How to password protect and encrypt a workbook
This document explains how to require a password to open a workbook which also encrypts the file.
To Encrypt an Excel Document with a Password
CAUTION: If you forget your password, the file can not be recovered!
1) With the spreadsheet you want to protect open, click File > Save As.
2) Give the document a name, as always.
Hint: If you already saved the sheet without encryption, and don't want to leave a plain-text copy on your drive, use the same name. This overwrites the plain-text version, and generates a warning, which is safe to ignore in this case.
3) Click Options.
4) In the Password to open box, type a password, then click OK.
5) In the Confirm Password dialog box, type the password again, then click OK.
6) Don't forget to save! Like any change in a document, password protection is not applied on disk until you save.