If the zoom level is fixed, the zoom-in or zoom-out gestures might not work.
If you can see the 
icon if you try to zoom in or zoom out of the document, it means the zoom level is fixed and you can't zoom in or out of the page.
To unfix the zoom level, press the icon and then the icon will be changed to 
.
Now you can change the zoom level of the document again.
Related Article
Fix/Unfix the document zoom level(Android only)