docx4j includes code for creating a PDF from a docx using iText. It can also use POI to convert a doc to a docx.
There was a time when we supported both methods equally (as well as PDF via XHTML), but we decided to focus on XSL-FO.
If its an option, you'd be much better off using docx4j to convert a docx to PDF via XSL-FO and FOP.
Use it like so:
wordMLPackage = WordprocessingMLPackage.load(new java.io.File(inputfilepath));
// Set up font mapper
Mapper fontMapper = new IdentityPlusMapper();
wordMLPackage.setFontMapper(fontMapper);
// Example of mapping missing font Algerian to installed font Comic Sans MS
PhysicalFont font
= PhysicalFonts.getPhysicalFonts().get("Comic Sans MS");
fontMapper.getFontMappings().put("Algerian", font);
org.docx4j.convert.out.pdf.PdfConversion c
= new org.docx4j.convert.out.pdf.viaXSLFO.Conversion(wordMLPackage);
// = new org.docx4j.convert.out.pdf.viaIText.Conversion(wordMLPackage);
OutputStream os = new java.io.FileOutputStream(inputfilepath + ".pdf");
c.output(os);
Update July 2016
As of docx4j 3.3.0, Plutext's commercial PDF renderer is docx4j's default option for docx to PDF conversion. You can try an online demo at converter-eval.plutext.com
If you want to use the existing docx to XSL-FO to PDF (or other target supported by Apache FOP) approach, then just add the docx4j-export-FO jar to your classpath.
Either way, to convert docx to PDF, you can use the Docx4J facade's toPDF method.
The old docx to PDF via iText code can be found at https://github.com/plutext/docx4j-export-FO/.../docx4j-extras/PdfViaIText/