I will try again to make it clear in plain English. 1. An undo is different form editing.; 2. An UNDO is the next click to a wrong click or wrong command executed wrongly; an edit is to go back to edit mode and then edit to correct the wrong click.

I will try again to make it clear in plain English.

1. 1. An undo is different form editing.;
2. 2. An UNDO is the next click to a wrong click or wrong command executed wrongly; an edit is to go back to edit mode and then edit to correct the wrong click.
3. 3. I tried to explain it with a calculator process.  1+123 =124 ; but it if you clicked 1+124 (instead of 123) you can CE (instead C which cancels everything) and correct 124 into 123..You are back in the game. The CE button in  calculator is equivalent to UNDO command in software like PRO DC
4. 4. Let me explain my specific difficulty. I have a 200 page document and and there are many invisible objects in many pages, I have to remove those.
5. 5. I removed all the objects in 199 pages ; but on page 200, I made a wrong click and deleted an object which should not have been deleted.
6. 6. Perhaps you can explain what I should do now.(step by step).

  7. If I had an UNDO command at that stage, I was ok. I could UNDO, get the object back and remove the correct object and then save the document.