(DWG or DXF) into PDF?
Thanks
www.lowagie.com/iText/
Check out iText for creating PDF documents. I have used it in a project before and it works great for most things.
AutoCAD files are proprietary though so there really is nothing out there, (for free at least) that can automagically convert DWG to PDF.
I would consult Autodesk and see if they have a Java API for what you are describing.
Otherwise the best solution I can think of is make the CAD drawers save there files in PDF format, because that can be easily done using AutoCAD.