> The pd4ml class will open and show my simple html file when I press GO.

It is a good sign. It means PD4ML works and renders HTML (in the case not to PDF, but to Image output device). I guess it also produces PDF to a TMP directory, but it cannot start a viewer application for some reason.

The problem with Tomcat still has no good explanation. I would recommend to try to configure a progress monitor:

It should help us to determine at which conversion phase it fails.