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
To access your current or prior year tax returns sign onto the TurboTax website with the userID you used to create the account - https://myturbotax.intuit.com/
Scroll down to the bottom of the screen and on the section Your tax returns & documents click on Show. Click on the Year and Click on Download/print return (PDF)
See this TurboTax support FAQ for accessing prior year online tax returns - https://ttlc.intuit.com/community/prior-year-return/help/how-do-i-access-my-prior-year-return/01/270...
After a return has been efiled and accepted, it can sometimes take the return PDF a while to update and become available at the Tax Home landing screen. If you can't use the Download/print button at the Tax Home or the "Download/print the PDF" link in the section "Your Tax Returns & Documents", OR if you're getting incomplete documents when you use one of those methods, here's an alternate way to print by opening the filed return back up to print at the Print Center.
NOTE: When you open your return back up, do not make any changes in it if it's already been filed. If you later have to amend the return, it has to start off exactly as it was when it was originally filed.
NOTE: Remember all PDF tax documents and tax data files are very sensitive files, since they contain your personal ID info, financial data, and possibly bank account numbers, etc. Be sure to store them safely and securely to guard against computer theft, hacking, etc.