I use PrinceXML for converting long HTML into PDFs and haven’t had trouble with large documents, though I don’t know if we’re in the same ballpark in terms of f

I use PrinceXML for converting long HTML into PDFs and haven’t had trouble with large documents, though I don’t know if we’re in the same ballpark in terms of file size or element count. It’s expensive but is a one-time purchase (and I think it’s free to use personally and to evaluate). You can also use it indirectly through DocRaptor (basically a PrinceXML SaaS with an API), though I’ve never tried it.