OK, I found a intermediate solution for that.

You need shell access for the following:
(or move it somewhere)

finally run: All must be done as root.

The result will be that you can click on a pdf from opera or filemanager, and the old (!) pdf-viewer will come up again.

Evince is still working and can be started from the menu.

This is not ideal, but despite some hacks I tried with these files, I couldn't get evince to automatically open the files.