HTML to PDF conversion using Chrome pdfium
PDFium (and PDF.js) are merely PDF Viewers, you cannot convert HTML to PDF with them.
The actual PDF generation in Chrome is done by the printing component using Blink (the rendering engine) and Skia (the graphic stack). There is an effort going on to decouple the printing functionality from the printing UI so that third-party projects can use the printing functionality (crbug.com/311308), but it is still work-in-progress.
Chrome 59 and up can be used to convert HTML to PDF, via the --print-to-pdf flag. See crbug.com/603559 for the specific feature itself, and https://chromium.googlesource.com/chromium/src/+/lkgr/headless/README.md for documentation about headless Chromium in general. This example loads example.com and outputs the result to `path/to/file.pdf.
chrome --headless --print-to-pdf=path/to/file.pdf https://example.comIf you want to know how to use Chrome's built-in PDF generator in JavaScript, follow the following question (which has not been answered yet): Javascript call programmatically the "Save as PDF" feature of Chrome dialog print.