A dead simple way to send files between two Linux machines not on the same network is to use a utility called [Magic Wormhole](https://github.com/warner/magic-wormhole). It is typically included in the standard repositories and is so simple the this blog post is going to end soon.
You can also rest in the fact that your data is secured by a [Password-Authenticated Key Agreement (PAKE)](https://en.wikipedia.org/wiki/Password-authenticated_key_agreement).