Qt assistant fails to open .pdf

I am not able to view PDF files included in a help collection. When I press links to the files, nothing will happen. The problem appears to be that launchWithExternalApp() in helpviewer.cpp will assume a PDF can be opened internally, and therefore do nothing, but the calling code is not actually able to display it.

If I apply the attached patch, everything works better in that launchWithExternalApp() will not recognise the file as one the assistant can open, and thus pass it to an external application so that it will actually be displayed. It is, however, not 100% clear to me whether this is the way PDFs are supposed to be displayed, or the Qt assistant itself is supposed to be able to handle them.