There are 2 possibilities if in case you can't see the undo icon.
If you didn't input any data
The undo button is not always exposed and appears after data such as handwriting, text boxes, and images are entered on the page.
If you go to the file manager after entering data or exit the app and return to the document, the undo button will disappear.
If you are in 4-page view mode
The Undo button does not appear in page 4 view mode.
To undo in 4-page view mode, use the undo gesture (double-tap with two fingers) on the page you want to undo.