The author is using Mosaic, available free to CompuServe subscribers. A plain text file can be downloaded and viewed by Mosaic itself. You will probably need to change the font size to fit the file on your screen. The author selected View, then Options, then Fonts, then 'Example', then Change Font, then Courier New 9 point (OK OK). This fitted a file like VERNAM.TXT into the screen space.
It is not for him to tell you how to run your web browser, but this gives you an idea.
Against each text file's link is an estimate of the length of that file, rounded to the nearest kilobyte, so you know what you are letting yourself in for if you download that file. No estimate is given for HTML files, but then none of these are very long. There is also a second number, which tells you how often a file has been downloaded from CompuServe, which is a relative indication of its popularity.
At the end of the text file, you will need to click on the Mosaic 'Backward' button or its equivalent. There is no 'Return to previous page' to click on. This is an argument for converting the .TXT file to HTML, but that will take a long time!