Using the zoom controls
Use the zoom controls, Zoom out control button and Zoom in control button, (located at the bottom of the document window) to enlarge or reduce the image on your screen.
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Note  In Windows, you can only zoom in to 300% when Enlarge window contents to improve readability is selected in the General tab of the Preferences dialog box, or if your system is using a HiDPI display.
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Tip  If you can’t use the zoom controls, it is likely that a script has been performed that locked the zoom level. To restore the zoom feature, run a script that has the Zoom Level script step with the Lock option turned off.
Note  You can also access the zoom controls by choosing View menu > Zoom In or Zoom Out.