A tax data file with a file extension of .taxyyyy, where yyyy is the tax year, eg .tax2019 for tax year 2019, can only be opened using the TurboTax desktop software installed on a Windows or Apple personal computer.
The online system has closed and I highly doubt your phone will download a PDF that large.
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)
The Turbotax App for any smart device is a portal to access Turbotax online. You can start, modify or complete your return on any computer with a web browser or your smart device app using the same login information.
The smart device app is not the standalone Turbotax program that can open a .tax file. That program is only available to install on a Mac or PC.