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 Snipping Tool (the Snippers) button, you zoom it and zoom out the PDF page in view. Otherwise, you zoom in and zoom out your selected rectangle area. The snipping tool button turns up whenever you complete Zoom In or Zoom Out.
Hot keys for Zoom In and Zoom Out,
Insert - Zoon In
Delete - Zoom Out
In Continuous View mode you can also use + for Zoom In, and - for Zoom Out.