DITA Open Toolkit Architecture 
  
  DITA Open Toolkit is an open-source implementation of the OASIS specification for the Darwin Information
    Typing Architecture. The toolkit uses Ant, XSLT, and Java to transform DITA content (maps and topics) into different
    deliverable formats.
  
Processing structure Map-first pre-processing DITA-OT provides a map-first pre-processing option as an alternative to the       default preprocess operation. The method, which was introduced in DITA-OT 2.5 as an experimental       feature, has since been improved and is ready for use in production scenarios. Map-first pre-processing provides       the same functionality as the default preprocess, but takes a different     approach. Processing modules Processing order Store API – Processing in memory Pre-processing modules preprocess target calls the entire set of steps.HTML-based processing modules PDF processing modules PDF  (formerly known as PDF2 ) transformation process runs the     pre-processing routine and follows it by a series of additional targets. These steps work together to create a     merged set of content, convert the merged content to XSL-FO, and then format the XSL-FO file to PDF.