You need a PDF reader to read it.
1) if the file downloaded with the file extension ".tax2018" then that is the wrong file...you downloaded the datafile...go back and find the PDF file.
2) if the file extension is ".txt" then edit the filename to be ".pdf" and it should then read/open OK with a normal PDF reader. A PDF download that comes down with a ".txt" extension is problem with some EDGE browsers...either change the extension after downloading, or use Chrome or IE11 to download the file instead.