Palm PRE User Manual page 24

Stop scrolling: Tap or drag the screen while scrolling.
Zoom gestures
Zoom in/pinch out to increase the size of items on the screen in Email, Web,
Photos, Doc View, PDF View, Google Maps: Place two fingers on the screen
and spread them slowly apart.
24 Chapter 2 : Basics
Zoom out/pinch in to decrease the size of items on the screen in Email, Web,
Photos, Doc View, PDF View, Google Maps: Place two fingers on the screen
and bring them together.
Zoom in or out a fixed amount (Web, Photos): Double-tap the screen.