What file did you download? Was it a .tax or .pdf or .txt file? If it downloads as a .txt file, try changing the .txt to .pdf or use a different browser to download it.
If you are using Microsoft Edge see
https://ttlc.intuit.com/community/troubleshooting/help/why-is-my-pdf-downloading-as-a-txt-file-in-microsoft-edge/01/27534