I have enabled the extension PDF Handler and I think it is not working. My test file at wiki page mh:wpe:Global Voting Platform#Project Brief is not showing as an image view but just as a link. An example of how it should look a pdf file like image view is here: https://sandbox.semantic-mediawiki.org/wiki/Show_PDF_files_like_image_files.
See also comments of Dmehus and Reception123 in https://meta.miraheze.org/wiki/User_talk:Driolar#Extension%20PDF%20Handler.
The suggestion given in T3530 is not useful, since PdfEmbed is not activated in my case.