Meanwhile I found out that KOReader is targeted for touch devices, bad luck. There is already an issue on GitHub concerning support for non-touch devices:
https://github.com/koreader/koreader/issues/1480
It is written there that for non-touch devices there is koreader/kindlepdfviewer project. Similar to kindlepdfwriter is librerator but I didn't find any information if it is possible to run it on my PocketBook 912.