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
