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.
DocRaptor - As the only API powered by the Prince HTML-to-PDF engine, DocRaptor provides the best support for complex PDFs with powerful support for headers, page breaks, page numbers, flexbox, watermarks, accessible PDFs, and much more
DocHub - Edit, send & sign PDFs online for free.
PDFCrowd - Pdfcrowd is a Web/HTML to PDF online service. Convert HTML to PDF online in the browser or in your PHP, Python, Ruby, .NET, Java apps via the REST API.