There's nothing built in at this time which does this for you.
Since a spreadsheet is basically a table, going to an XHTML table, and from there to PDF (using http://code.google.com/p/flying-saucer/
), is probably a sensible approach.
Or you could go from XLSX to a docx table, and use what is in docx4j already to convert that to a PDF.
Or you can go XLSX to FO to PDF (which is what we do for docx). Or you could use iText...
One limitation to be aware of: xlsx4j won't do calculations for you, so if your spreadsheet contains these, you may be better off using a different library (eg POI or LibreOffice), since you'll probably need to use that to perform the calculations first.