I'm afraid you have your answer. The only way to convert a .tax file to a PDF is with the Turbotax program.
However, there may be free solution if you wait 9-10 months. Some time after Turbotax online closes for 2017 in October, the company will make a PC amend-only version as a free download, for people who originally used TT online and now need to amend. It usually comes out in December or January. If you wait for the free amend-only download of TT 2017, you can import your tax data file and say you want to amend. The first step in amending is to print a copy of your original return for your records, so that allows you to print or make a PDF. Then you can quit without actually amending.
(If you have a tax data file for an earlier year, like 2016 or 2015, those amend-only versions are available now.)
You can also get a transcript from the IRS for free, or a photocopy for a fee (the fee is more than buying a second copy of TT, though.) https://www.irs.gov/Individuals/Get-Transcript
There are also companies on the web that maintain old computers with old versions of tax software, and will convert a tax data file to a PDF for a fee. I can't recommend one and don't know how secure and private they are, but one of those companies might be able to make your PDF for less than the cost of a new copy of Turbotax.
