So I saved my 2018 file, but my PC can't open it without a suffix, that is, .tax2018 must have a further openable suffix. What is it?

[DoninGA]

The 2018 tax data file which has a file extension of .tax2018 can only be opened by the TurboTax software for tax year 2018.

TurboTax 2018 CD/Download editions - https://turbotax.intuit.com/personal-taxes/cd-download/

Download the PDF of your return for review.

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