PDF , Process.Start, .
:
Uri uriDownload = new Uri("http://myReportServer?MyReport&rs%3aCommand=Render&rs:Format=pdf");
string strSavePath = @"C:\test\test123.pdf";
System.Net.WebClient wcli = new System.Net.WebClient();
wcli.DownloadFile(uriDownload, strSavePath);
System.Diagnostics.Process.Start(strSavePath);
UPDATE:
, wcli.DownloadFile():
wcli.Credentials = new NetworkCredential("username", "password", "domain");
wcli.Headers.Add(HttpRequestHeader.UserAgent, "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1)");