So, for the benefit of those of you not yet initiated into the world of preferences files, or if you just want to expand the horizons of your knowledge, I offer this tutorial. The topic is so big ...

If PowerPoint says that it can't open or read this type of presentation file, then use these proven fixes to resolve the issue once and for all.

If you cannot open an Excel file and see Excel cannot open the file because the file format or extension is not valid error, see this post.