I created a PDF viewer QML plasmoid (custom C++/QML component that uses poppler library) and now I would like to create some kind of a configuration dialog for changing the PDF file path. By default there is already a configuration dialog and I think it would probably be the best solution to somehow extend this to include my custom configuration items.

I read some thing about configuration xml files and also about the custom ".ui" file for the configuration dialog but I didn't get anything useful from that (I doesn't really tell you what and how you can do this).

So ... what is the best solution for having some GUI configuration items for a QML plasmoid?

You may wish to ask about this on the plasma-devel@kde.org mailing list. I have previously worked with Plasma applets, but that was with QGraphicsView based applets - which are different I believe.