Have a look at "2D to 3D conversion" in the Help files. You can import a "sketch picture" as a tracing aid to create sketch geometry, but PDFs are not supported. You'd have to convert to .bmp, .gif, .jpg, .jpeg, .tif, or .wmf formats to use as a sketch picture.
If you have Adobe Illustrator, you can open the pdf's as vector art then save as DXF's or even as illustrator files and SWx can open them. If not, jack your screen resolution up as high as it will go, open the pdf, zoom to what you want, then take a screen shot and paste it into Paint (Start>All Programs>Accesories>Paint). Save it as your facorite format, then take Art's suggestion of using the resulting picture as a "sketch picture" to trace on top of.
If you are lazy and (maybe) want to spend a lot of time cleaning up data, try Wintopo ( See recent thread) I admit the following is not fair or current - I haven't used Wintopo for the better part of this decade. Back in 2001, it provided a DXF file, but it sucked - all sorts of gaps, and lots of repair was required. Since then I always traced because then I have control. On the crest of turning to 2008 maybe Wintopo is still free (don't know because I don't care to use it again seeing that I have Illustrator) and maybe it is excellent (don't know because I haven't tried it in 6 years) I don't even know if it works with pdfs now - it was always bitmpas in the past (you know the difference between vector and bitmap art, right? If not, respond, and we will let you know about it - it's sort of confusing but everyone needs to know that it matters) So maybe it will work with you. If not, the first paragraph will definately work. ED
I've had many people tell me they couldn't get CAD information from vendors. Before you get too involved in tracing or raster to vector conversions it might pay to contact the source of the pdf files and ask (nicely) if they have a dxf or dwg. Chances are the pdf was made in CAD. Sometimes you have to do a little creative digging but frequently you can find the guy with the file. And if they get enough requests it may turn up on Condiment Central.