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Do you mean PDF to DWG or inverse?
If you mean DWG to PDF, there is a DWG to PDF printer since Autocad 2007 or was it 2006. That's the same for DWG to DWF.
For the PDF to DWG you have to pay I'm afraid. But why don't you ask your provider of CAD drawings a DWG-file? The PDF to DWG-file won't be very usefull anyway, I guess, because the result will be like a dxf-file (all geometric objects and texts) (that's what I read on a website from a converter)
Anyhow, success in your search and if you'll find one for free, please let us know.