I tried to put together below a set of frequently asked questions and useful links about the DITA Open Toolkit.
What is the DITA Open Toolkit?
The DITA Open Toolkit is a publishing tool used to convert DITA content into various output formats. It's official web site, containing download links and documentation can be found here: http://www.dita-ot.org/.
How do I use the DITA Open Toolkit?
Besides this, there are applications which come with the DITA Open Toolkit bundled. For example Oxygen XML Editor comes bundled with both DITA Open Toolkit 1.8 and 2.x. Oxygen provides visual means to run the bundled DITA Open Toolkit using a concept called transformation scenarios.
What version of DITA Open Toolkit should I use?
Probably most of the DITA implementations available are still using DITA Open Toolkit 1.8.5. This usually happens because companies have output customizations which have not yet been modified to work with the latest DITA Open Toolkit releases.
But if you do not have legacy plugins or customizations you should try to use whenever possible the most recent stable version of the DITA Open Toolkit available on the official download page.
What outputs can I obtain using the DITA Open Toolkit?
The entire set of default available output formats is available here: http://www.dita-ot.org/dev/user-guide/AvailableTransforms.html. But the DITA Open Toolkit can be enhanced by installing plugins to provide additional output formats.
What is the general architecture of the DITA Open Toolkit?
The DITA Open Toolkit is a quite large mixture of ANT build scripts, Java libraries and XSLT scripts. It has a pipeline-based architecture which uses plugins to publish DITA content to various output formats. Most of the DITA Open Toolkit customizations that you want to make in order to add new publishing capabilities or to customize existing publishing choices can be made without modifying its internal core.
What is a DITA Open Toolkit plugin?
A DITA Open Toolkit plugin can either provide a new publishing format, customize an existing publishing stage or provide a DITA specialization vocabulary. The plugin can use one of the numerous extension points available in the DITA Open Toolkit: http://www.dita-ot.org/dev/dev_ref/plugin-extension-points.html.
How do I customize the HTML-based outputs?
There are a number of parameters which can be set to customize the HTML-based outputs:http://www.dita-ot.org/dev/parameters/parameters-base-html.html. For example you can specify your own CSS stylesheet to be used with the generated HTML output.
You can also create a plugin to customize the HTML outputs by adding a custom XSLT stylesheet: http://blog.oxygenxml.com/2013/12/creating-simple-dita-open-toolkit.html.
How do I customize the PDF output?
The PDF output is obtained by passing the original DITA content to XSL-FO and then generating PDF using an XSL-FO processor. The default bundled and used XSL-FO processor is the Apache FOP but you can also install separately and use commercial PDF processors like Antenna House or RenderX XEP.
There are a number of other solutions for obtaining PDF from DITA: http://blog.oxygenxml.com/2015/11/possibilities-to-obtain-pdf-from-dita.html.