You could check out the LEADTOOLS Document SDK to achieve this functionality.
There is the RasterCodecs Class that can load the PDF file (and other document formats) as SVG using the LoadSVG method and then you can use the LEADTOOLS Vector SDK to take the SVG file and convert it to DXF.
Here is a forum post on the LEADTOOLS Technical Support Forums that has a sample project showing how to achieve this:
How to Load PDF as SVG then save to DXF
Here is the relevant information from the post:
This sample will load a PDF (or any of the Document Formats we currently support) as an SVG file and save it to a temporary file, then load the temporary SVG file and save it as a DXF file (or any of the Vector Formats we currently support).
The application is written in Managed C++ (CLR Console Application).
It uses our .NET interface to load the Document Format as SVG and save the SVG file to a temporary file on disk.
It then uses our C interface to load the SVG file from disk and save it as a vector format, in this case: DXF.
Disclaimer: I am an employee of the company that wrote this library