Zoom In 
and Zoom out 
allow you to enlarge or reduce the view of your PDF document. Zoom In works like a magnifier, and Zoom Out does just the opposite.
If you have not pressed down the Cropping Tool button, you zoom it and zoom out the PDF page in view. Otherwise, you zoom in and zoom out your selected rectangle area. The cropping tool button turns up whenever you complete Zoom In or Zoom Out.