If you are trying to open the tax data file, which has a file extension of .taxyyyy, where yyyy is the tax year of the file, eg .tax2018, that file can only be opened by the TurboTax software for that year.
The Adobe Reader can only open files with an extension of .pdf, not the tax data file.
To access your current or prior year online 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)
Let's assume that you have a file that ends in .pdf (DoninGA is right that other filetypes are probably not PDF).
First, make sure that your Adobe Acrobat Reader is up-to-date. Please see https://helpx.adobe.com/acrobat/kb/install-updates-reader-acrobat.html for instructions on how to update both Adobe Reader and Adobe Acrobat.
Note that it can happen that TurboTax is creating PDF according to a newer standard than your version of the Adobe software will support. Yes, PDF evolves over time, and sometimes newer versions of PDF can't be read with older versions of Reader.
Second, your file that you think is a PDF may not actually be a pdf file. If you can open the file in Notepad (I am assuming that you are a Windows user), then the first few bytes should be %PDF-1.X where X is a single digit (like 4). If this is not true, then you may not have a PDF file at all.
Third, if you do think that you have a PDF file, you can try to repair it using one of many 3rd party tools to repair the PDF file. Look at this post on an Adobe forum for references to a number of 3rd party tools (I am sure that some are free) that you can use to try to repair the files - https://forums.adobe.com/thread/1826105
If I don't have turbotax online, can someone open a 2017 file?
@swimmer1 wrote:
If I don't have turbotax online, can someone open a 2017 file?
A tax data file created by TurboTax for tax year 2017 can only be opened by the TurboTax software for 2017. The tax data file will have a file extension of .tax2017. A PDF of a tax return can be opened by any compatible PDF viewer, such as Adobe Reader.
What exactly are you trying to do?