Add the PDF URL website to have 'Insecure origins treated as secure':.Go back to the site and Reload the page.
That being said, you should be able to do some workaround to allow it (for this site only, as completely disabling the feature is certainly not advised.) But it's a bit difficult to guide you very specifically without having the context how the actual URL of the PDF looks like (and on what site it's loaded from.) The party to fix the issue is the website owner/developer, not you.