How to send password to encrypted document sending by email, if we only have recipient email address?

In the situation where you don’t have any means to establish a second channel of communication (i.e., the recipient refuses to define or establish one), the less insecure way to deliver the decryption password by e-mail is by sending it through a separate e-mail, making sure this email does not have any indication of its content (e.g., do not use in the subject or body words like password, archive, cryptography, etc.), and it is not sent just after the e-mail with the encrypted file.

A more complex, and secure option, would be sending an e-mail with an invitation for a meeting (e.g., using free versions of virtual meeting applications), and after confirming the identity of the recipient, send the password as a message, or file. This alternative would take advantage of the encrypted communication channel between you and the recipient to provide a greater level of security.