There are three places that you are able to undo/redo your changes

A) Toolbars in the Document Viewer.  Tap the undo button to undo, or long press the undo button to bring up both undo and redo.

 B) Thumbnails Viewer 1.  Tap the Thumbnails icon from the bottom-left corner of the main screen. If you don't see the Bookmark icon, tap the center of your screen to bring it up.

2. Tab on Edit button to show undo/redo buttons:  

  Note that the undo/redo stack is restricted to the edit pages actions (such as rotate, delete, duplicate, etc.) in the Thumbnails Viewer.