PDF is based on PostScript, which is a Page Description Language (PDL) also from Adobe. The first PostScript printer, the Apple LaserWriter, launched the desktop publishing revolution. The interesting thing about PostScript is that it is a full programming language, with loops and conditionals and so on. When Adobe designed PDF, they kept the same imaging model as PostScript, but stripped out the programming language. Thus they ended up with a dynamic page description language (PostScript) for media (printers) that cannot fully take advantage of a dynamic PDL, and a static PDL (PDF) for media (i.e., computer screens) that could have benefited from a full programming language!