[Enginerrrd]

Not great, you can only officially export PDF, PNG, or SVG, though their cloud management does seem to work really well and as I understand, files are stored locally on each device once synced.

That said, it runs linux so you could conceivably backup the files yourself. You can ssh into your tablet, run sftp, w/e. But they are in some proprietary .lines format.

There's a wiki with more details (the file format stuff is further down): https://remarkablewiki.com/tech/filesystem

It may be possilbe you could hack it to run xournal, but I've no idea.