RoxyApps eBook Conversion Tool - Convert djvu, epub, fb2, lit, azw, lrf and other eBook files to txt, pdf, pdb, rtf and other formats
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.
Foxit Reader - Foxit Reader is a free and light-weight multi-platform PDF document viewer.
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
DjVuLibre - DjVuLibre: Open Source DjVu library and viewer
PDFShift - Convert any HTML documents to high-fidelity PDF using a single POST request