The Yellow Paper is a formal definition of the Ethereum protocol, originally by Gavin Wood, currently maintained by Nick Savers and with contributions from many people around the world.
It is free culture work, licensed under the Creative Commons Attribution Share-Alike (CC-BY-SA) version 4.0.
The latest version is generally available as a PDF at https://ethereum.github.io/yellowpaper/paper. This version of the yellow paper can be used to get links that direct to specific info in the yellow paper, which is useful e.g. for pedagogical and academic purposes. Here is an example of such a use.
The paper also comes as a single latex file Paper.tex, which is built as a PDF as follows.
git clone https://github.com/ethereum/yellowpaper.git
cd yellowpaper
./build.sh
This will create a PDF version of the Yellow Paper. Following building, you can also use standard pdflatex tools like http://latex.informatik.uni-halle.de/latex-online/latex.php for compiling/preview.