WebClient.DownloadString results in mangled characters due to encoding issues, but the browser is OK
It's not lying. You should set the webclient's encoding first before calling DownloadString.
using(WebClient webClient = new WebClient())
{
webClient.Encoding = Encoding.UTF8;
string s = webClient.DownloadString("http://export.arxiv.org/api/query?search_query=au:Freidel_L*&start=0&max_results=20");
}
As for why your alternative isn't working, it's because the usage is incorrect. Its should be:
System.Text.Encoding.UTF8.GetString()