It doesn't seem to be anywhere in the documentations themselves, nor on the website. Google is failing me, and I can't access IRC currently.
What makes the nice looking documentation pdf for the SensioLabs products?
The docs are built using Sphinx, a documentation tool on top of the reStructured Text format.
This tool allows one to compile the source as a HTML output, resulting in the docs shown on symfony.com/doc. It also has a PDF builder, which outputs the docs in the PDF files.