The TurboTax program has been using the .taxnnnn (where "nnnn" is the tax year) filename extension at least since I started using the program back in 2003. So no, that format has never changed in the 15 years I've been using it.
Also, the program provides the ability to save the tax return in PDF format too. Every tax year once my completed tax return has been accepted and processed by the IRS and I'm basically done with it, I always save the .taxnnn file *and* a PDF copy of that file, and burn them to CD so that I have them if, lord forbid, I ever need to access a prior year's tax return in the future. In fact, I have the .taxnnn file and the PDF file from every year since 2003 all on one CD. That way, if I just need to "see" a prior year return, I can look at the PDF copy with any PDF reader program such as the free Acrobat Reader.
ALso, since TurboTax only suppors their program for the current tax year and three years back, I will also download the most recent update for that program "just before" it's support life cycle ends, burn it to CD and keep it with my TurboTax CD for that tax year. Then if I ever need to change or amend a return I don't worry about it if it's more than three years back. I've got the updates and corrections on the manual update file I downloaded/burned to CD.
So for example, if I actually need to "fix" something on my 2003 tax return because of an IRS audit today, I can install the program, then install the updates, and actually "fix" it. Of course, the IRS would never audit a return from 15 years ago, unless they have reasonable cause to suspect intentional tax fraud on my part.
As for your tax file that don't have the PDF or .taxnnnn filename extension, I don't know what they are. Could be anything.
