Use the Edit menu Undo command to undo your last action in the viewer.
You can undo the following types of action.
Last single keystroke
Cut
Paste
This command is disabled when a command cannot be undone; that is, you have not previously performed an action in the viewer that can be undone.
To undo your last action, perform one of the following actions
Select the Undo command from the Edit menu
Press Ctrl+Z
Your last single action is then undone.
You can use this command to undo only the last action that you performed. You cannot undo a sequence of previous actions.