What kind of file is it downloading? A .pdf or .tax2019 or a .txt?
If it downloads as a .txt file, try changing the .txt to .pdf or use a different browser to download it.
Are you are using Microsoft Edge? The solution is log out of TurboTax, then switch browsers to Chrome or Firefox, log back in and download your pdf file again.
Before filing, you can preview the 1040 or print the whole return
https://ttlc.intuit.com/community/accessing/help/how-do-i-preview-my-turbotax-online-return-before-f...
How to get a copy after filing
https://ttlc.intuit.com/community/prior-year-return/help/how-do-i-get-a-copy-of-a-return-i-filed-thi...
