If you find that you can only scroll between pages horizontally, and you can't turn pages/flip through a notebook vertically, horizontal scrolling direction is most likely enabled.
From the document view, tap the More (...) icon in the top-right corner > Scrolling Direction and 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.