Most modern browsers have a shortcut that makes it easy to page up/down in a browser window: the space bar. You can use the space bar to page down and Shift + space bar to page up.

This tip is mostly oriented towards people using laptops. If you are using a desktop, or a laptop with a keyboard and mouse connected, there are easier ways to do this (like the scroll wheel on the mouse, or page up/down buttons on the keyboard). But I found this much easier for paging on my laptop.