To do this, choose a suitable way that is as separate as possible from the one you use to send the file. So you could send the password by messenger, the file by email. Sending the password verbally, by phone or by mail is also a good way to avoid compromising the protection of your file. Avoid sending the password and the file in the same message (such as a single email) – this lowers the level of protection because a potential attacker would only need to intercept a single message to gain access to the file.