If you've not seen my résumé, that is what this post is about, and you should probably look at it first to get an idea of what's happening.
PDF and ISO9660 are two very different file formats. PDF is actually a surprisingly human-readable (but, sadly, not very human-writable) plain-text format, technically descended from PostScript. PDF is not very forgiving on changes, as it stores tables of offsets to various elements, so it is very difficult to insert or remove content manually without going through a robust PDF manipulation library.
ISO9660 is a disk filesystem, meant to be written once and read many times on many different kinds of hardware. As it is sector-based, it expects data structures to be at particular absolute offsets.