I was searching for NPM package which could help me to combine and convert multiple PNG images into pdf format. However I could only find pdf to png conversion packages not the other way around.
https://www.npmjs.com/search?q=png%20to%20pdf&page=1&ranking=optimal
PDF is just a document format. You can eventually add new image. Look, here is a simillar question. Is there an npm module to modify a pdf file in node.js?
And the first link from this topic https://github.com/bpampuch/pdfmake#images
Lately I've started using imagemagic, it's pretty fast. There's a node package for imagemagic available out there although I haven't tried it yet.