I'm using Doxygen and I'm pretty much fine with it.
It supports chm as well as pdf generation or just HTML documentation.
I simply type my comments in a special way, and Doxygen will make documentation out of it.

Don't know if it really works for a big project, though.
Always depends on your need.

If you want to test it, in my contest project (A3tools) I included a short documentation made with Doxygen. It was done in a few hours and thus is somewhat incomplete.
However you can compare to the source code and see how comments need to be written in order to generate documentation from it.

In this case it is intended as developer help, so it contains listings of all files, but this can be configured so you get a documentation for customers as well.