I understand the difference between a PDF and .tax file. I prefer to save my return both ways. Every time click on the "download .tax file" my computer downloads a PDF. My return is filed already. I can get to the screen you referenced if I elect to amend my return, however the same thing happens, I click on the download for the .tax file and it downloads a PDF instead of a .tax file. Tomorrow, I am going to try logging into a different computer and see if I can get it it download a .tax file. If so, I can email it to myself and then save it. I really think the problem lies in the fact that I somehow have told my computer to use Adobe to open .tax files, but I can't figure out how to undo that.
