Portable Document Format is a file format that is primarily associated with Adobe. It is used as a way to represent two-dimensional documents in a display resolution- and device-independent fixed layout format. It is based from PostScript, an earlier Adobe product that was more focused on printer resolution.
PDF in Mac OS X
The PDF format, while being an ISO-certified standard, is mostly proprietary to Adobe since the most sufficient way to gain access to Adobe's implementation of PDF is by paying Adobe royalties. This is also the same with PostScript.
In the days of NEXTSTEP, NeXT Computer payed Adobe royalties in order to use Adobe's implementation of PostScript in NEXTSTEP's next-generation window system, Display PostScript. However, when NEXTSTEP was bought by Apple and eventually rewritten as Mac OS X, the decision was made to drop Display PostScript and to reverse-engineer Adobe's implementation of PDF (which had many features and advantages over PostScript in text and graphics rendering) in order to avoid paying Adobe royalties.