YAGS is licensed under the GNU Public License, and is considered to be Free Content, in much the same way that Linux is Free Software. This means a number of things.
The point of the GPL is to give you, the gamer, certain rights. Firstly, you have the right to download the game for free. You can print out as many copies as you want, and distribute it however you want. This even includes selling it.
What you can't do is claim copyright of it, or change the license. You can sell copies, but those copies are still licensed under the GPL so those you sell it to are free to make their own copies and distribute those for free.
Secondly, you have the right to see the source code. In the case of computer software the source code is pretty well defined - it's all the bits required to compile binaries of the software. For YAGS, the source code is the text (XML) files on which the PDFs are based, and the stylesheets which are used to generate those PDFs. It also includes all the associated icons and images.
Thirdly, you have the right to modify the game in any way you choose. Since you can access the source, you can change it and generate your own PDFs (or Kindle books, or HTML files or whatever you want) as long as they too are licensed under the GPL.
Getting the Source
The source for YAGS is available on GitLab. You can browse it there, or checkout your own copy using the following:
git clone https://gitlab.com/samuelpenn/yags.git
Unless you like reading XML files though, it's not the best way to read the rules. These files are very easy to process using a computer program however, so if you wanted you could load all the skills into a database or a spreadsheet for example.
It's also possible to format the XML into a more human friendly format. The project that this is part of, Yagsbook, is designed to do just that. Yagsbook is a formatting engine that turns XML which describes a roleplaying game into a readable PDF.