Weirdly, when I visit the extension URL directly at
https://addons.opera.com/extensions/download/pdf-viewer/
it installs the extension but leaves it disabled. When manually enabling it, it reports:
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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?
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So, something is causing Opera to think the extension is originating outside the permitted extension repository.