The Browse Object
Or, Find comes up every time I hit Page Down!
Those double arrows at the bottom of the vertical scroll bar are not really page up/page down arrows. They really mean Browse Next and Browse Previous. The little dot in the middle lets you set the Browse Object, so that you can use those double arrows to go to the Next and Previous for any of 12 different categories.
The default setting for the Browse Object is Page—but whenever you do a Find, the Browse Object automatically changes to Find so that the double arrows become Find Next, Find Previous. You can also use the Browse Object to record macros for Next Heading, Previous Heading, or any other setting that you find yourself using frequently to navigate your document.
If you really want Page Down and Page Up, you can reset the Browse Object to Page when necessary. Or you could use a keyboard shortcut—there is probably already one assigned in Tools>Customize.

Click and hold the little circle, and the menu will pop up—drag to make a selection. As you highlight each icon, the bottom line will tell you what each icon means. (This screenshot was done on a Mac, so may appear unfamiliar to Windows users—the essential elements are all there, however.)