If you are trying to open a 2015 tax data file, which will have a file extension of .tax2015, that file can only be opened by the 2015 TurboTax software installed on your computer. If the file is a PDF it can be opened by any compatible PDF viewer, such as Adobe Reader.
If you used the TurboTax online editions to complete the 2015 tax return -
To access your current or prior year tax returns sign onto the TurboTax website with the userID you used to create the account - https://myturbotax.intuit.com/
Scroll down to the bottom of the screen and on the section Your tax returns & documents click on Show. Click on the Year and Click on Download/print return (PDF)
See this TurboTax support FAQ for accessing prior year online tax returns - https://ttlc.intuit.com/community/prior-year-return/help/how-do-i-access-my-prior-year-return/01/270...
If you used the desktop CD/Download editions installed on your computer, the only copy of your tax data file and any PDF's will be on the computer where the return was created. TurboTax does not store online any returns completed using the desktop editions.
Or go to this IRS website for free federal tax return transcripts - https://www.irs.gov/individuals/get-transcript
For a fee of $50 you can get a complete federal tax return from the IRS by completing Form 4506 - http://www.irs.gov/pub/irs-pdf/f4506.pdf