In Microsoft Word, one can track changes.
How do I do so in Powerpoint?
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You can make track changes in PPT 2010 but not in 2007.
Suppose you have 2 presentations PPT A and PPT B where changes have been made in PPT B.
You can also hover over the pencils to see what's changed. The Next and Previous buttons are like in Word where you can navigate to each change and accept or decline the change or accept/decline all changes.
In PPT 2007 you can only track inserted comments rather than compare any changes between 2 presentations.
This http://geigercomputers.com/track-changes-in-word-and-powerpoint/ also has similar answer but in more details.
Easiest way is to do track changes manually - put a line through text you think should be deleted and make the font on added text red. It's easier than screwing around with comment boxes and compare features.
If Dante wrote the Inferno today, he'd add another circle of Hell for Microsoft product developers.