PowerPoint Compatibility Issue with Windows Version

I have a PowerPoint slides that sum up to 7MB. I have to reduce the size to a much smaller one. I cannot convert it to .JPG as I have a co-author that I need to consider. The problem is, they used Windows 97-2004 while the slides are created in PowerPoint 2008 for Mac. But I am saving the slides as 97-2004 for compatibility issue. But even if I can open the presentation on my computer, if I send it to others, they are unable to open it. What seems to be the problem? Please help me fix it. Thanks.
