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)
