Microsoft can’t retrieve forgotten passwords.
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.
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.
Follow these steps to encrypt an Excel file:
Click File > Info > Protect Workbook > Encrypt with Password
When you/another user tries to open the file, the following screen appears: