There are 2 annoyances:
- Nxtpaper uses at the bottom a massive hight bar;
I solved that by using gestures
- PocketBook seems to use top and bottom bars;
I solved that by PocketBook > Settings > Page by uncheck of "Status bar at bottom" and by PocketBook > Settings > Display by uncheck "Status bar in immersive mode" and by uncheck "Navigation bar in immersive mode";
Then select PocketBook "Full screen"
This provides me with the maximum screen size.