How do I create a working .dwg file from a paper copy of a drawing that I scanned and saved as a PDF?

Well... this will be tricky...

Assuming it's a relatively simple line drawing you could take that image and run it through a RasterCellCoercer, have that export polygons. Dissolve those based on their cell values and then run the result through a CenterLineReplacer.

Best case scenario is that you'll end up with vector lines conforming to the ones on the original drawing. You would most likely still have to do a lot of cleanup to make this a workable CAD file.