The HTML version uses jsMath and JavaView to display math fonts and images, respectively, while the PDF versions contain links to JavaView versions of most of the figures. Be warned that some of the JavaView files are large and may run slowly on some hardware, and that the PDF files are roughly 5 MB each.
The first PDF version contains soft links to the figures, and is therefore suitable for reading with a browser plugin, whereas the second contains hard links to online figures, and is therefore suitable for loading directly into an appropriate PDF viewer. In either case, you must be online to follow the links.
(You can in principle make a local copy which you can read offline by downloading the first PDF version, the HTML files it refers to, and the source files called by the applets in those files; you will also need your own jsMath and JavaView installations. Be warned that the resulting configuration does not appear to work properly under Linux; this appears to be a bug in Acrobat under Linux.)