The memory is mostly lost to time but MacOS X before 10.4 (released in 2005) took PDF screenshots of the desktop.

I think that pissed a lot of people off, ha ha. But if you had multiple displays .... multi-page PDF. PNG still can't do that. :-)

Yeah, there was a switch to PDF from Display PostScript at the time of the NeXT merger.

Speaking as an iOS developer, we use PDF files to represent our vector icon assets in the app.

I thought it was a bit odd of a choice on the platform vs SVG which seems more popular (and which the Android devs on our team use), but CoreGraphics using it internally ties the whole picture together.