I started the free version but bought Turbo Tax Deluxe. Why can't I download a PDF file for my records? I click on Download .tax file and its a tiny file not usable.

The tax data file (.tax) can only be opened by the TurboTax software for the same year as the data file.  You should be downloading the PDF of your return to view or print the return.

To access your current 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 and on the section Your tax returns & documents click on Show.  Click on the Year and Click on Download/print return (PDF)

Or -

When you sign onto your online account and land on the Tax Home web page, scroll down and click on Add a state.  See screenshot

This will take you back to the 2018 online tax return.

Click on Tax Tools on the left side of the online program screen.  Then click on Print Center.  Then click on Print, save or preview this year's return.  Choose the option Include government and TurboTax worksheets