MJML - The open source framework for responsive emails
pdflayer - Free, powerful HTML to PDF API supporting both URL and raw HTML conversion. Unlimited document size, lightning-fast and compatible PHP, Python, Ruby, etc.
BeeFree - BeeFree is an all-in-one solution that works anywhere you are.
PDFShift - Convert any HTML documents to high-fidelity PDF using a single POST request
Elite Email - Drag & drop email builder for awesome newsletters
Tcpdf - TCPDF: Open Source PHP Class for generating PDF documents