Quartz's model is compatible, there is even a 1:1 mapping of many PDF concepts to quartz, but it isn't really PDF.

There is a PDF renderer in a higher layer which makes PDF convenient for content, and I there is a "to PDF" capture mechanism to reuse drawing code for PDF generation.

I think Quartz did a look around to see what graphic models were surviving the test of time and chose one rather than inventing their own. Rumor was they wanted to use Display Postscript, but Adobe wanted royalties so they made an API mapped to the concepts of PDF instead.

Interesting. Apple specifically says it's "built on PDF"; in fact, Steve Jobs said so at Macworld San Francisco 2001.

Not only that but NeXT computer's display model was based on PostScript. Apple moved on to PDF once Adobe released the specs.

Can't find any information on PDF being the base for OSX UI, I'd appreciate any links.

"Quartz's feature-rich drawing engine leverages the Portable Document Format (PDF) drawing model and offers Mac OS X applications professional-strength drawing functionality."

http://developer.apple.com/graphicsimaging/quartz/