I was just now surprised to find that the list returned from CGImageSourceCopyTypeIdentifiers() does not include PDF. I swear it used to! This was testing on OS X 10.7.4.
CGImageSource's PDF support has always been a bit funny. You're able to create a source from a PDF just fine. And creating thumbnails from that works a treat too. But any attempt to create a non-thumbnail image will fail. I assume this is because there is no definitive way to rasterise an arbitrary PDF.
So in a way it makes sense to not include PDF in the list of supported types. But I wish I'd come across it in a release note somewhere, and that the CGImageSource docs covered PDFs. Filed as rdar://problem/12574361