It depends on how the PDF file was created. I use VcarvePro and when a PDF only contains "bitmap" data they will not load. If it contains vectors they work fine.
If someone was giving me a file to cut I would much rather have it in the correct PDF format than DXF. Every PDF file I ever successfully loaded had the correct dimensions of the parts to be cut. Most DXF files I find on the internet are way too big and have to be re-sized.