Ai files converted to default pdf...how do I reverse?

I have some very important AI files on my hardrive.  I was searching for them and clicked on one and was asked to convert to pdf default.  How do I change this.  I need to send the AI files as AI files not PDF.   I have Ai Files on a memory stick, but when I search for AI files on the memory stick they come up as PDF.  Anyone know how to do this?