You can perform both the Undo and Redo functionality easily in PDF Architect.
Undo lets you annul the last action that was just performed. There are two easy ways for you to do this. In the top left corner, above the File menu, you will see the backward arrow indicating the Undo feature:
You can also perform this function by holding down the CTRL key on your keyboard and then choosing the Z key.
Redo allows a user to redo anything that was previously undone with the Undo feature. You can use this feature by clicking on the forward arrow next to the Undo arrow. It will be grayed out if you have not already used the Undo feature.
You can also Redo by holding down the CTRL key and then also choosing the letter Y.