Weirdly, when I visit the extension URL directly at
it installs the extension but leaves it disabled. When manually enabling it, it reports:
The extension "PDF Viewer" was added from outside the official add-ons website. It will require the following permissions:
Access your tabs and browsing activity
Access your data on all websites
Do you confirm the installation?
So, something is causing Opera to think the extension is originating outside the permitted extension repository.