How to use (magic) trackpad scrolling to scroll only one page up/down with Preview?
I dont have a Magic Trackpad, but on my Magic Mouse its the same gesture that is used for forward and back in Safari. In the mouse its swipe two fingers to the left and right, so if the Trackpad is just like that of a macbook, Im 99% sure its swipe three fingers to the right or left.

Hope it Helps!
Kevin