7-Zip can password protect and encrypt any file you put in a zip folder, regardless of the file extension.
Navigate to the 7-Zip download page.
Click the correct .exe for your operating system, and save it to your computer.
Click "Install" then once it's finished, click "Close."
Find the file you want to encrypt, right click it, navigate to 7-Zip>Add to archive...
You will be presented with this screen.
Change the archive format to "zip" to make your zip folder.
Create a password for the document, re-enter it, then change the encryption method to AES-256, then press "OK."
A zip folder will then be created in the directory your file was in that is both encrypted and password protected with the password you gave it.
NOTE: If you lose or forget a password, Excel cannot recover your data.
NOTE: Never email the password to the file in the same email. Either relay the password over the phone or send it using other means like a text message. Do not email the password to the recipient.