I move, add, delete thumbnails in the preview on left. Often I must undo or redo a change but no edit undo exists for this. I have to hope I haven't hit "save" or else I have no chance of recovering a deleted page. Or if I've moved a page, I have to remember where and manually move it back, as no "undo" and "redo" exists for this purpose. If you could "undo" and "redo" for editing thumbnails (and not just the content of the document), it would be fantastic.