There is an excellent opensource app koreader
https://github.com/koreader/koreader/wiki
I am using it on kobo and I am very happy. Though on kobo I use the native version whilst on Onyx I had to try the android build which was horribly slow. I wonder if it was only a performance problem related to Boox max or the android version is far from being optimized. I am going to check the koreader forum when I have few minutes to dig into it.