Are you sure that you did not download the tax data file and not the PDF?  The tax data file will have a file extension of .taxyyyy.  Where yyyy is the tax year, eg .tax2018, and can only be opened by the TurboTax software.  The PDF download will have a file extension of .pdf.

To access your current or prior year online 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)

Why is my PDF downloading as a .txt file in Microsoft Edge?

If you're using TurboTax Online, you may experience Microsoft Edge saving the PDF version of your tax file as a .txt file.

Solution

There are two ways to solve this issue:

1. Change your browser (to Google Chrome or Firefox, for example) and save from there.
2. Replace the .txt extension with .pdf.

In the EDGE download...those are really just PDF files, that's why they look funny.

After downloading with EDGE....just change the file extension to ".pdf" and then they will open as normal PDF files.  (Don't know why some EDGE systems do that).