If you find that you can only scroll between pages horizontally, and you cannot turn pages or cannot flip through a notebook vertically, Horizontal Scrolling direction is likely enabled.
From the Document view, tap the More (...) icon in the top-right corner > Scrolling Direction > change to Vertical.
You can also access Document Editing from the Gear icon in the top-right corner of the Library view > Settings, or from the More menu > Document Editing > Scrolling Direction.