In Portrait mode, zoom out to the point where you can see the entire page on screen (with empty space around it).

If you then zoom in one level, the height of the background page fits the entire screen perfectly, but the width of the background page is wider than the screen, requiring you to pan left or right.

Can that zoom level in Portrait mode be set to be the max width of the screen. Presumably this will leave a little bit of blank space vertically, but at least this view you would give you the maximum zoom whilst still seeing the entire page.