Author here, if you have any questions, feel free to ask them.
It really is amazing how hard it is just to retrieve the currently opened pdf file and its page number in a pdf viewer. Some pdf viewers (Like Zathura) provide this via DBus, but even very common ones like Evince don't. I managed to find a way using gvfs, although it's a bit of a hack.
For others (e.g. Mendeley), I have no idea on how to do this... Anybody have ideas? it is Qt based, maybe I can hook into that via some debugging tool?