https://wiki.gnome.org/GnomeGoals/NewDocumentationInfrastructure

This patch should work but I couldn't test-build it because of bug 704654.

I'm not sure how to handle src/PdfMod/Gui/Actions.cs though. I've not really worked with Mono before but I think just calling
Process.Start ("help:pdfmod");
should be sufficient.

Behind the scenes, yelp-tools installs the help to

/usr/share/help/C/pdfmod/index.docbook

instead of

/usr/share/gnome/help/pdfmod/C/pdfmod.xml

I think the help:pdfmod link will work as long as the help is installed to one of $XDG_DATA_DIRS or $XDG_DATA_HOME as help/C/pdfmod/index.docbook.

I don't think yelp will load docbook from an arbitrary directory but loading Mallard works fine assuming you have an index.page. (maybe that's a Yelp bug...)

Anyway I decided not to touch the file but I think you should be able to significantly simplify the help handling.

Once the build is fixed, you should be able to run
$ jhbuild buildone pdfmod
$ jhbuild run pdfmod
or jhbuild run yelp help:pdfmod

and then you don't have to worry about running help from the source directory.