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