Is there a way to find out who were the last few (3-5) people who
modified a PowerPoint file? PowerPoint's Properties screen may give
the name of the last person, but there is no history. I'm thinking
along the lines of a binary editor/decoder or forensic software.

I realize the Journal feature in Outlook may reveal this, but I
don't have access to the computers that modified the files. I will
also contact OnTrack to see if they have a way of doing this.

You should state what version of windows your using what type of file system.
Are they all using one computer or what.

That's a good point, and the answer is I don't know. But my client
just went directly to OnTrack, which said they could probably find out
the last 5 or 6 users who accessed the file (though they might not
necessarily know what the users did). I'm thus going to cancel this
question so nobody wastes time on it.