The only way to open a TurboTax .tax file is with the CD/Download TurboTax software for the corresponding year. For example, to open a .tax2019 file you need the TurboTax software for 2019.
If you really have a .tax file, not .tax20xx, that is a very old file, from 2007 or earlier. TurboTax files for 2008 and later have the year included in the file name extension, such as .tax2019.
