A free tool called DiskInternals ZIP Repair was able to recover a corrupted PowerPoint file (*.pptx) that I had on my computer. I learned that pptx files are actually zip files in disguise, and so using a utility to repair a corrupt zip archive could work. At first I thought it was one of those sketchy bloatware programs, but Lifehacker has written an article about them before, so it should be fine.
Using the program was pretty simple. I just opened it and told it which zip file (in my case the pptx file which I renamed to a zip file), and then the program did all the work and recovered everything. It was like magic!
Nevin Fradian October 7, 2016
Users can also try Kernel for PowerPoint file recovery software to recover inaccessible data from corrupt or damaged PPT/PPTX files.