I didn't think PDF had transparent background. Every time I need transparency on an import to SketchUp, it's had to be PNG. I have used a lengthy process in Photoshop for turning white background to transparent, mostly for scanned hand drawn artwork, but it's a bit of a chore.
DWG out from SketchUp has segments for arcs in 2D dwg, but real arcs in 3D dwg. That's why you're advised to use that latter when going to CNC software. I've wondered if PowerCADD could leverage that too.