PDF.js Express does not support document manipulation, so this is not possible.
Manipulating PDF documents is a very complex process, especially when you’re trying to convert HTML.
If you need this functionality you will have to upgrade to PDFTron Webviewer, which has document manipulation.
Adding HTML into a PDF is extremely complex, but it involves converting your HTML into a canvas, transforming the canvas to fit the document, converting the canvas to an image and then inserting that image into the PDF.
If you decide to go this route, you can contact PDFTron Tech Support and they will be able to help you more than I can.