PDF started as a pure (100% ASCII!) page description language, but it later was transformed to a binary format, and even later, accessibility support made it easier to retrieve the text from a PDF document (before that, all you would have were the glyphs. For English text, these often mapped 1:1 to characters, but they did not need to) and some support for reflowing documents was bolted on (http://www.adobe.com/uk/epaper/tips/acr5reflow/).

Both accessibility and this reflowing do require participation by the creator of the file, though.