The application works perfectly in local environment but when I deployed it in the online test site, the PDF generated is only a blank page with the evaluation note in the footer.

Of course PD4ML is able to work in local environment as well as in the Internet.

Obviously there are some network-specific issues on your side. I would recommend to try to dump HTML content before it is passed to PD4ML (to make sure the source HTML is not blank).

Also pd4ml.enableDebugOutput() forces PD4ML to dump external resources URLs it tries to load.