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 a free culture work, licensed under Creative Commons Attribution Share-Alike (CC-BY-SA) version 4.0.
If you just want to read the paper, the latest version is generally available as a PDF at https://ethereum.github.io/yellowpaper/paper.pdf. If you find that the borders for links block too much text when viewing the PDF in the browser, you can instead download it and open and view it with a PDF viewer application such as Adobe Acrobat or Evince, where the borders are less likely to display over text.
However, if you want to edit the paper, then read on. The paper comes as a single latex file Paper.tex.
It is recommended to use an IDE such as Visual Studio Code with the LaTeX Workshop extension, to edit the tex file, and show the PDF.
Another option is to separately edit the tex file and build as follows (you'll still need to clone the repo then open the yellowpaper folder):
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 for compiling/preview, like http://latex.informatik.uni-halle.de/latex-online/latex.php.
You can use TeX Stack Exchange; https://en.wikibooks.org/wiki/LaTeX/ (e.g. Bibliography Management and Hyperlinks); and BibTeX editor.
The previous protocol versions are listed in BRANCHES.md.