Way one: send the password separately, through a one-time link
Protect the file as usual, with a PDF password or an encrypted ZIP or 7z archive, and send the file by whatever channel the recipient expects. Then paste the password into the creator above and send that link by a different route. The password is encrypted in your browser, the first reveal destroys the stored copy, and the file and its key are never in the same message. This takes seconds and needs no account.