Tcpdf - TCPDF: Open Source PHP Class for generating PDF documents
PDFShift - Convert any HTML documents to high-fidelity PDF using a single POST request
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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.
Docamatic - Easily generate high quality PDF documents from HTML or templates. Fast, scalable and privacy focused PDF generation API.
PDF-LIB - Create and modify PDF documents in any JavaScript environment