Giving error if we enable CloudFlare

The PDF has to reside at the same domain as WordPress is installed. If you offload PDFs to a CDN, then the URL will not match. For security reasons, browsers restrict cross-origin requests initiated from scripts and thus it gets blocked.

There is no work around unless you want to build your own plugin.

https://github.com/mozilla/pdf.js/wiki/Frequently-Asked-Questions#faq-xhr