Free Document Conversion API: Docverter
Docverter is an open source application that combines multiple tools inside a jRuby app and works as a document conversion server.
It can be installed to Heroku (the easiest option) or locally and be used with the help of an HTTP API.
The application can be used for converting multiple types of documents including HTML, reStructured Text, Markdown and more as the input and lots of popular formats as the output (PDF, HTML, Docx, EPUB..).
Documents that are converted can be grabbed instantly or Docverter can place them inside Amazon S3 so that you can get them later.
It can be functional for many scenarios including generating PDF invoices with HTML, providing the download of articles in multiple formats and more.