Thank you for your question. I like to show my PDF and brochure inside Fancybox POP-UP window. It will not work to show POP-UP window and also a PDF viewer. I’m not sure but there is a lack of information when you try to show PDF inside my browser.
It looks like you’ll need to create a local version of the ‘viewer.html’ page to load the actual PDF file (notice in the Codepen example they link to https://mozilla.github.io/pdf.js/web/viewer.html rather than directly to the PDF file.
I had a quick look and there’s a relatively simple version of the viewer page at: https://github.com/mozilla/pdf.js/blob/master/examples/learning/helloworld.html you can see on line 20 where they link to the PDF file