A. Toolbars in the Document Viewer
Tap the Undo button or long-press to bring up both Undo and Redo.
B. Thumbnails Viewer
1. Tap the Thumbnails icon from the upper-right corner of the main screen.
2. Tab the Edit button. Note that the Undo/Redo is restricted to the edit pages actions such as rotate, delete, duplicate, etc. in the Thumbnails Viewer.