Rendering engines are software tools that convert XML source into PDF documents or other deliverable document formats. The rendering engine most commonly used with the DITA Open Toolkit is Apache FOP, which ships as part of the toolkit. However, the Open Toolkit can alternatively use other engines including RenderX XEP and Antenna House Formatter.

Rendering engines are not designed to work directly with individual XML schemas such as DITA. Instead, rendering engines process documents supplied in a standard page layout XML formatting language: XSL-FO. In order to take advantage of rendering engines, content must be converted to XSL-FO format first. DITA content can be converted to XSL-FO easily, using XSL-T (an XML closely related to XSL-FO).

To transform a DITA document into PDF through a rendering engine, the content is first processed by an XSL-T processor (using rules encapsulated in an XSL-T file) to produce an XSL-FO version of the document. That XSL-FO document is then processed into a PDF file using a rendering engine. In some ways, rendering engines are like Adobe Distiller, which transforms PostScript files into PDF documents.

Rendering engines are sometimes referred to as PDF generators, or XSL-FO processors.

Apache FOP shouldn't be confused with a plug-in to the DITA Open Toolkit known as the FO plugin, which is an extension to the Toolkit to allow it to be used with the RenderX XEP rendering engine.