How to include PDF of rulebook in VASSAL module help menu.
Perform these steps at the very beginning of a "brand new edit session" of your module, i.e. right after you first open your module for editing. This will avoid an intermittent Vassal bug where you are prevented from saving your module.
(1) Put your PDF in a directory on its own on your hard drive (e.g. in a "help" subdirectory of where you keep your original art files, etc).
(2) Right click the Help Menu and Add HTML Help File.
(3) For the Menu Entry, type "Rulebook" or whatever you want
(4) Set the "Contents" to the directory (on your own hard drive, not in the Vassal vmod/zip file)
(5) Set the Starting Page as the pdf file name.
(6) Save module and you should be good to go.
If you need to update the PDF file later with a newer version, it is much easier to do this manually (i.e. by opening your VMOD file as a ZIP file, and inserting the new version of the PDF file to the right location) rather than try to repeat this procedure.