I have tried using the 3.5 version of the binary but the AvalonEdit.Sample totally fails to run displaying the message AvalonEdit.Sample has stopped working. The problem appears to be with the DropDownButton.cs class as it is missing the public access modifier. I assume that the class is mean't to be public. Anyway making the change in source code and re-compiling for .Net 3.5 fixes the problem for my requirements but I thought I should advise you in case anyone else has the same problem with this version of the download from Nuget.
Regards pianoboy