Page Up/Down on Adobe reader on mac
After opening a new pdf the cursor doesn't really exist and depending on your settings Left/Right arrows work properly for Page Up / Page Down.

As soon as you move the cursor into the document you will navigate inside your pdf and Page Up / Page Down won't work anymore with the Left/Right arrow keys.

To restore the previous behavior just click on the dark grey background.
That's a bit difficult if View -> Zoom is set to 'Fit Width', because the gap between document and frame is very small and hard to hit with the cursor.