PDF is a meta format which may contain raster data as well as vector data.
Raster data is obviously not of much advantage for geometry creation in a vector based CAD application but can be used for manually tracing over the drawing. Additionally, raster data could/should be saved in a raster format as e.g. PNG anyhow without the need of wrapping a PDF envelope around.
Regardless of this, the free Adobe Reader allows to export a PDF via "File > Save as... > Image..." to a raster format in the desired quality/resolution etc.
For converting vector data inside a PDF free Inscape (or commercial Adobe Illustrator; Corel Draw probably too) can be used to save as a vector-based DWG/DXF.