Convert PDF to Autocad DWG with Free Online Service

Just go to this link, then click Browse, select your PDF file then enter your email address.

You may have to dodge a few ads in the resulting email, but the actual DWG output is fast and the quality is not too bad.

I have posted about extracting useful data from PDFs before:
Obtain Vector Information from PDFs using Free Tools

Or if you want to convert an image to a DWG file:
Converting a raster image to a DWG file