You need to edit the .desktop files for Opera & qpdf2. They can be found in /home/QtPalmtop/apps/<tab_name> where <tab_name> is the name of the tab where the program icon appears (unless you added your own tabs with the tab manager, in which case the <tab_name> is UserTab#).
When editing the .desktop file, you will need to remove \"application/pdf\" from the \"MimeType=\" line in the Opera file and add it to the same line in the qpdf2 file.
I find that the application ZAAM is handy for editing .desktop files -- it eliminates the need to hunt for them.
Qtopia must be restarted (or, at the very least, the Launcher refreshed) in order for the changes to take effect.
