Boox Color Series User Manual page 97

Fit to Canvas and Fit to Screen
When the canvas is larger than the screen(1x1), you can go to Toolbar /
Zoom to adjust the canvas size to "Fit-to-canvas" or "Fit-to-screen" .
Fit-to-canvas: the canvas will be automatically expanded to fit the whole
screen. When the canvas is larger than screen, you can long press the
canvas to pan it around.
Fit to screen: the canvas will zoom out to fit to screen. You cannot write
on the grey area.
Fit the Canvas
Fit on screen