The VirtueMart Project manages the documentation in the DocBook format. You can checkout the sources in the DocBook format and transform the DocBook source using an XSL Transformer into PDF, HTML, CHM or whatever else... All you have to do is checkout the directory /trunk/documentation from dev.virtuemart.net (details see above).

The easiest and most professional software to modify DocBook sources is the XML Mind XML Editor. After you have downloaded and installed the software you can just open the main documentation file and start working on it: