That's where I am right now. I wanted to try the -DWITH_PDF=ON to see what additional functionality that provided. I don't know for sure, but I think it might be used for pdf thumbnailing or other ancillary pdf processing functions. The current kpdf works great with kdegraphics built without -DWITH_PDF=ON, so I'm not driving myself crazy over it (I've already done that long ago :)
However - what sucks is I'm the poppler-qt3 maintainer for AUR, but I'm NOT the AUR maintainer of Qt3 - issues.....
Here we go.
1) yes it is the thumbnailing plugin that it is used for, nothing else, unfortunately if you disable it, you loose all of your kfile-plugins.
2) make your own package, like trinity-qt3 ( i think i own that one, but you can have it) that way you can muck around with it, and give it what we need.