WebClient - The remote server returned an error: (403) Forbidden

Check if the server, you are trying to access is set up to use improved TLS protocol. Make sure to add this to Global.asax.cs

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;

I've just tried it with Fiddler running to see the response and it returns the following notice with the status code.

Scripts should use an informative User-Agent string with contact information, or they may be IP-blocked without notice.

This works.

    WebClient webClient = new WebClient();
    webClient.Headers.Add("user-agent", "Only a test!");

    string content = webClient.DownloadString("http://he.wikisource.org/wiki/%D7%A9%D7%95%D7%9C%D7%97%D7%9F_%D7%A2%D7%A8%D7%95%D7%9A_%D7%90%D7%95%D7%A8%D7%97_%D7%97%D7%99%D7%99%D7%9D_%D7%90_%D7%90");