Adobe converted all my PPTs to PDFs, without me changing anything

I don't k now how this happened, but Adobe has converted every saved PPT on my computer as a PDF. I don't even need to open the file - it just systematically went through my computer and converted all my saved files. The format on all my presentations are now a complete mess. How did this happen and how do I undo this? I didn't change the default program to open everything as a PDF, or use Adobe to open my files.  This isn't as bad as iTunes deleting all the non-apple music files on my computer, but it's up there.