Can you tell me how to download my 2012, 2013, and 2014 tax returns via PDF? Every time I attempt this, it provides only a corrupt .txt file, not a pdf. Please advise.

If the file ends in .txt instead of PDF, that is a known issue that can happen to some folks.   Try changing the file extension from .txt  to .PDF.   Then try opening the PDF with a PDF reader or a browser.   If that doesn't work, then try downloading it using a different browser.

@magnusonx 

There was a typo in my comment above.  I mistakenly typed .tax.   It has now been corrected.  i.e., try changing the file extension from .txt to PDF.   Or try downloading with a different browser.