You can't explode a PDF, it's a raster-based format. There are third party programs out there that will convert PDF's to DWG's, but the process tends to be ugly at best. Google PDF to DWG for more info.
It's not exclusively raster-based. Try plotting a drawing to PDF, and then adding it to a fresh drawing as an xref. Now, try to draw a line using endpoint snap from one of the lines inthe PDF. Magic? No, it's the vector component of PDFs.
So, given that, there *should* be a way to explode.
The snaps depend on the origin of the pdf - dwg to pdf will allow snaps but a pdf derived from a scan for example will not have vector information..
Of course it depends on the source; the PDF is a container which can hold compressed raster and vector. The thing is AutoCAD is already recognizing the vectors (this surprised me greatly the first time my snaps took to it). Therefore the only reason we cannot currently "explode" (bind/burst) the PDF appears to be down to autodesk limiting the functionality. What we have is a block/group that prevents exploding.
Is this a security matter? Possibly. The vector output of linewidth in a PDF usually results in two endpoints at each end in order to represent the thickness. Text is *often* exploded to paths. Sometimes, lines are simplified (generalized in compression). Curves are broken into segments, which in the wrong hands could imply certainty about the accuracy of the data.
My guess is that the explode/import functionality is being shelved until they can resolve the matter of ownership/security. When you can create a vector PDF and choose for it to unexplodable, then we might see the functionality appear (but there's still an incredible wealth of drawings in PDF form that will be insecure)
As long as we've moved onto work-arounds:
Inkscape is a free open-source alternative to Illustrator. http://inkscape.org/
Just open the PDF, and save as DXF.