For a Touch-enabled device - Pinch (move two fingers toward each other) or stretch (move two fingers away from each other).
Using mouse & keyboard - Hold down the Ctrl key and scroll the mouse wheel up/down.
Using keyboard only - Hold down the Ctrl key and press the Plus ('+') or Minus ('-') key.
Using mouse only - Click the Plus ('+') or Minus ('-') button in the lower right corner of the document window.
With the first update of Adobe Reader Touch for Windows 8 (released on February 1, 2013), we added the Plus and Minus buttons for easy Zoom control for mouse users. The Zoom buttons become visible only while you are moving the mouse on the screen. This also works with a trackpad.