Encrypt a Microsoft Excel Workbook
If you have a MS Excel Workbook that contains sensitive information, you can password protect the workbook so that it can be safely transmitted via email or stored in a network file share. Be careful to not lose or forget the password as the workbook cannot be opened without it.
To encrypt and password protect your workbook, follow these steps in Microsoft Excel
Open the doc you want to protect.
Choose File > Info > Protect Workbook.
Select Encrypt with Password.
The Encrypt Document dialog box appears; type your password into the text box and repeat when prompted.
Save your document - it is now encrypted and password-protected.
If you're sending the protected file to someone via email, be sure to provide the password to the recipient through a means that is not email such as a phone call or text message.