Actually, it has a webapp which you can acess via your browser over the wireless or USB connection ip, which allows you a rudimentary upload/download :
https://support.remarkable.com/hc/en-us/articles/36000266133...
That said, there are whole lot of sync tools that users have created which work very well with Linux (for example, I personally use remarkablefs, which is a Fuse based filesystem to mount your remarkable)
https://github.com/reHackable/awesome-reMarkable/blob/master...