I've built a SaaS service that generates PDFs, and I've just added an HTML/CSS editor for this purpose [1]. It uses Liquid templates and SASS. After you set up all of the fields, you can just post data to a simple API. We also provide an online form that people can fill out.
If you just need to post your own HTML/CSS and get a PDF, then I'd recommend DocRaptor [2].
If you just need to post your own HTML/CSS and get a PDF, then I'd recommend DocRaptor [2].
[1] https://formapi.io/templates/tpl_mFAsZEmhCtG9FX5N/edit
[2] https://docraptor.com