Yeah, PDF can play embedded sound files. It is an overly complex format. Its character encodings are defined by embedded CMap files, which are a restricted subset of PostScript. It includes by reference the CCITT G3 fax standard, PNG, JPEG, JPEG-2000, JS, Deflate, TrueType, the PostScript Type 1 font format, a separate richtext format that isn't PDF for annotations (ISO 32000-1:2008 §12.7.3.4), and several other things. In different contexts it represents line endings as \r, \n, or your choice of either of those plus \r\n. And the 3-D models thing isn't an extension; it's in the base spec (§13.6), although the actual 3-D format is ECMA-363, which is included by reference, like Deflate and JPEG.

But it's nowhere close to the complexity of the entire web stack, not within an order of magnitude. It's much, much simpler.