Lets start with HTML to PDF:
Let me introduce you to Athena.
Using Electron, Athena creates print-quality PDFs in any paper size.
The Docker image supports every major language.
Aggressive mode allows you to convert high-quality docs even when CSS is damaged or missing.
Then for the PDF to HTML, let me introduce you to pdf2htmlEX
pdf2htmlEX renders PDF files in HTML, utilizing modern Web technologies. Academic papers with lots of formulas and figures? Magazines with complicated layouts? No problem!
pdf2htmlEX is also an online publishing tool which is flexible for many different use cases.
Some of the examples pdf2htmlEX are really impressive.