The .zip files are meant to be extracted onto a local hard disk for offline viewing.
However, The Parts Lists do not work from a hard drive on most modern browsers because browsers from 15 years ago weren’t quite as locked-down as they are today. For the technical among you, the files violate the Same Origin and CORS (Cross-origin resource sharing) Policies.
There’s nothing dangerous about the files, just coding anomalies that the browser doesn’t like.
However if you have a personal web server, these files will work perfectly because they are the same files served from my web server.