If you use a smartphone, you are familiar with this gesture, as well. On touch devices, you can easily enlarge or reduce the size of the display by spreading your thumb and finger apart or by pinching them together, respectively.
To enlarge the content display of your file, position your fingers on the screen at the point on which you want to zoom in and spread them apart.
To reduce the display of the content in your file (zoom out), put two fingers on the screen and move them toward one another in a pinching motion.
The screen reduces as you move your fingers on the screen.