1) if it has a file extension ".tax2018"...you made the wrong download selection.
2) if the download has a file extension ".txt".....then you made the right selection, but perhaps used the "EDGE" browser to download your file......for that situation you can just edit the file extension to be ".pdf" and it will open in the proper PDF reader. OR...just use a different web browser...IE11 and Chrome both download with the proper file extension.