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.
Apache PDFBox - The Apache PDFBox™ library is an open source Java tool for working with PDF documents.
PDFShift - Convert any HTML documents to high-fidelity PDF using a single POST request
PortableSigner - PortableSigner is a signing (with X.509 certificates) program for PDF files.
Tcpdf - TCPDF: Open Source PHP Class for generating PDF documents
DocHub - Edit, send & sign PDFs online for free.