Display and function of PDFs in a browser depends on a browser plugin. And if there is no browser plugin for PDF files installed, like on my Adnroid device, the user will not be able to see the content at all… I’d advise using a more cross browser/platform compatible way of embedding PDF files like the Google Viewer.
There is a method of integrating Google Viewer and (Easy) FancyBox described on https://demo.status301.net/easy-fancybox-sandbox/pdf-embeds/ (alt method 2) but basically you do this:
1. Go to Settings > Media and completely disable the PDF option.
2. Go to https://docs.google.com/gview and compose your document URL, then append it with &embedded=true
(important!) and use that to create a link in your post.
3. Then in the HTML view editor tab add class="fancybox-iframe"
to that link code so that it will be recognised by FancyBox.