An interesting aspect of PDFs that I didn't know until quite recently is that they're a subset of PostScript and that in fact accounts for some of the heftiness. PostScript is a full-on programming language (albeit an unusual one) but PDFs are not (i.e. they're not Turing complete). They do not support control flow and what could be expressed as a simple loop in PS must be unrolled and stored as a series of simple declarations/expressions for a PDF.

The advantage is that PDFs don't need a full program interpreter to be rendered.