How to disable the built-in PDF viewer in Opera

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.

I am posting this answer from Opera 43.0, which has recently updated. For me, the "Disable" checkbox for the Chromium PDF Viewer cannot be toggled off.

Instead, in Settings > Basic > PDF documents, toggle Open PDF files in the default PDF viewer application checkbox. With that, PDF files are not open in browser, but in your default desktop application instead.

Using Opera 46.0 the check-box instead is found under: Settings/Websites/PDF documents