Getting Buy Now for software I have already purchased

I purchased Acrobat many years ago from Adobe. Now when I open a document, I get a new dialog box saying I have to buy the software again. I have a valid product key and I called Adobe support and they said I have a valid key but needed to reinstall the software again to solve the problem. I don't have the media anymore and the tech said to come here and you would help me get installation media.