VectorWorks has this built-in to the program. However, I'm not sure if it's as good as a dedicated converter. It also depends how the PDF file originated. If it was created from the CAD application, it should have the vector data embedded in it, to allow conversion fairly easily and accurately. However if it was scanned, or created as a bitmap image, it's a bit harder. You'll need to use a bitmap tracing application. The free, open source multi-platform application Inkscape has a really good bitmap tracing algorithm, but there are other professional tools that can do it for you. This is harder and less accurate, and the resulting files won't necessarily be 'clean' CAD files. By this I mean that the lines may be made up of lots of little line segments, resulting in a large file that is much harder to use.
However, if you're trying to convert line items with a single vector design, then vector tools are probably more suited to you. If this doesn't apply, then the CAD application will need to import your file as a bitmap, and the scan itself will have to have the vector data embedded in it to allow the conversion. You'll need to trace your design with a vector tool. There are many them, all available as paid downloads. Unfortunately, it's not always that simple. You'll probably need a dedicated software program to handle the conversion; if the conversion is going to be in a vector format, then you'll have to convert the file to the necessary vector data. This is not ideal, since you'll need both your original document and the converted vector data along with it. There will be some issues with the resulting scan, the quality of which.