Export docx to PDF via XSL FO, using FOP
Up until docx4j v3.3.0, this was part of docx4j itself. From 3.3.0 onwards, it is an optional separate sub-project.
docx4j will use export-FO automatically if it finds it in your classpath.
v6.x series is the last which will support Java 6.
Since docx4j v3.3.0, the default PDF Converter is an eval version of Plutext's commercial renderer, which takes a completely different approach, to offer much better fidelity and performance. For more on this, please see https://converter-eval.plutext.com/client_java.html