I use this method described in a German forum (thanks @Elim Garak. See post number 6 for KOReader). It works for all file types, not only books. I also use it for sending epub with DRM (previously opened with an app with DRM support on the smartphone) which I then read with the Kobo epub reader. I don't use the computer at all.
https://www.e-reader-forum.de/t/mit-.../#post-2405047