MuPDF - MuPDF is a lightweight PDF and XPS viewer.
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.
Okular - Okular is a universal document viewer based developed by KDE.
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
qpdfview - qpdfview is a Qt-based tabbed document viewer using the Poppler library for PDF rendering and CUPS...
PDFShift - Convert any HTML documents to high-fidelity PDF using a single POST request