next to impossible - if the pdf was created through a vector program u can use illustrator and "livetrace" the file. but even still its not the most accurate.

Ive also herd of rastorcad....i dunno much about it though

rhino imports/exports these file types. not sure about the scale issue though...
as far as live trace in illustrator, there's no need if it's already a vector based pdf--just open it up, ungroup as needed, and edit. raster based would be another issue

open it in illustrator then export as dwg.

You can try this converter http://www.autodwg.com/pdf-to-dwg-converter/
as there is an option to set the scale of the conversion.

drop it in rhino and export as a .dwg file. keeps the lines clean and whatnot.

we need to see what you're working with for quality, resolution etc.

Yes ^ask for original dwg, you'll be surprised how very few people use acrobat to make technical drawings :)

Otherwise, import in Rhino if it has layer info and lineweights, if not go illustrator.