The resources at this site are available in several formats:
OpenDocument Format
Several resources are available as OpenDocument formats. These are true open standards and are widely supported by office suites and on-line services.
Postscript
I recommend the PostScript (.ps) files for printing, especially if you are on a Linux / Unix system. Also, I understand that Mac's come with postscript viewers. Windows users would need to get a special viewer.
PDF (.pdf) files are made for viewing with Adobe's acrobat reader. They are similar to postscript but users of MS Windows are more likely to have a PDF viewer than a postscript viewer.
HTML
HTML files are text files with web page markup. View them with your browser. For printing, PS or PDF files are better. For viewing on your computer, the OpenOffice.org format is better than the HTML version. (I make the HTML pages available mostly for the benefit of people googling for a specific person or event.)
Note on HTML and older versions of the encyclopedia: The software I used to convert the LaTeX sources into HTML has trouble with the order of the entries - on the index and contents pages, the entries are not in alphabetical order. I do not consider this a serious problem because it only affects those two pages and otherwise all the info is there.
LaTeX
Update - as per the front page, I am going back to LaTeX for the encyclopedia and many other JHDP resources. It is more powerful and more flexible when working with a project of the size and complexity of the JHDP encyclopedia. Word Processors are great, but I become less of a fan when my project gets up to 600+ pages full of chapters, headings, images, tables, indexes, cross-references, and bibliographic entries.
Until about summer of 2004, LaTeX files were the basic files from which I made all the others. They are just text files with special markup that the LaTeX programs use to convert to various file types. (No longer the case) Currently, all the LaTeX files have been converted to OpenOffice.org format and the LaTeX files / format is no longer used for the JHDP.
BibTeX
BibTeX files are used with LaTeX to keep flexible databases of bibliographic information and insert references where needed. Since the db is a text file, it is very useful, even if you aren't using LaTeX.