How to disable the built-in PDF viewer in Opera

Opera 29 comes with a built-in PDF viewer. PDFs are now opened with this viewer by default. How can I configure Opera to use an external plug-in (like Adobe reader) instead?

I could not find any preferences in Opera to configure how to handle certain mime types.

Solution

Opera's plug-ins are deeply hidden in a developer menu. To show them (in Opera 29.0) select the following:

Main menu > "More tools" > "Show developer menu"
Main menu > "Developer" > "Plug-ins"
Button "Disable" for "Chrome PDF Viewer"

This brings back the system's PDF Viewer.