Use of PDF
It is widely stated that Quartz "uses PDF" internally (notably by Apple in Quartz's early developer documentation), often by people making comparisons with the Display PostScript technology used in NeXTSTEP (of which Mac OS X is a descendant) and OPENSTEP. Quartz's internal imaging model correlates well with the PDF object graph, making it easy to output PDF to multiple devices.
Read more about this topic: Quartz (graphics Layer)