Pre-built AvalonEdit binaries are available in the
AvalonEdit NuGet package. The package contains binaries for .NET 3.5 and .NET 4.x.
The source code is available on GitHub.
For introductory documentation, please take a look at the CodeProject article "Using AvalonEdit".
The source code download of AvalonEdit contains sources for a Sandcastle help file. The compiled documentation is available on this website.