what type of file did it download? Also were there other document types as options for download?

It is a text document and it's just gibberish when I click on it.  No PDF or anything readable.  I've tried 22 times, so it just keeps changing the name of the file (1), (2), etc through (22)

It's probably the .tax2018 file?  You need the Turbo Tax program installed to open that.  You need to download the PDF file.
<a rel="nofollow" target="_blank" href="https://ttlc.intuit.com/questions/1912544-how-do-i-save-my-2018-turbotax-online-return-as-a-pdf">htt...>

I have two accounts, one for each son.  One account finally gave me a printable pdf, which I assumed I hadn't been able to get until the files were "accepted", so I waited on the other account until both returns were accepted but it is still just downloading a useless gibberish .txt file.

Try using a different browser to download it.  
Try replacing the .txt with .pdf.

Awesome!  It didn't give me the option to change the .txt, but switching browsers did the trick.   THANK YOU!

Solved
Were you downloading it with Microsoft Edge?  What did you change to?

Switching the browser worked for me! I had the same problem.