Note: This topic covers file-level protection only, and not workbook or worksheet protection. To learn the difference between protecting your Excel file, workbook, or a worksheet, see Protection and security in Excel.

Warning: 

• Microsoft cannot retrieve forgotten passwords, so be sure that your password is especially memorable.

• There are no restrictions on the passwords you use with regards to length, characters or numbers, but passwords are case-sensitive.

• It’s not always secure to distribute password-protected files that contain sensitive information such as credit card numbers.

• Be cautious when sharing files or passwords with other users. You still run the risk of passwords them falling into the hands of unintended users. Remember that locking a file with a password does not necessarily protect your file from malicious intent.

This article was originally published here.