Well, I don't know about PDF, but PostScript, which it's based on is basically just a programming language for drawing symbols in specific positions on a page. Depending on how it was written (or more likely, generated) this could be readable, or incredibly unreadable.
As an example, in post script the following example from Wikipedia would simply show the text Hello World:
%!PS
/Courier % name the desired font
20 selectfont % choose the size in points and establish
% the font as the current one
72 500 moveto % position the current point at
% coordinates 72, 500 (the origin is at the
% lower-left corner of the page)
(Hello world!) show % stroke the text in parentheses
showpage % print all on the page
Now if you're lucky you can just extract all quoted text and read those in order, but that's unlikely to work for all documents.