Opera 29 comes with a built-in PDF viewer. PDF files now open with this default viewer. How to configure Opera to use an external plugin (for example, Adobe reader)?
I could not find any preferences in Opera to configure how to handle certain mime types.
Opera plugins are deeply hidden in the developer's menu. To show them (in Opera 29.0), select the following:
PDF.
Opera 43.0, . "" Chromium PDF Viewer .
"" > "" > " PDF" " PDF PDF". PDF , .
Opera 46.0 "/-/ PDF"
Just check the following settings path
opera://settings/content/pdfDocuments?search=pdf