PPTX files shouldn't appear as being corrupted once they're saved. Open Microsoft PowerPoint and then go to File > Open and select your file from the list. You should have the possibility to open it this way. There are many apps on the market that pretend to fix these files, but they're very expensive and not reliable at all, that's why I recommend using PowerPoint to open it. This could also be a simple error that can be fixed by restarting PowerPoint.