GenerateReport.cs

public async Task<FileStreamResult> GenerateReport()
{

    CredentialCache credentialCache = new CredentialCache();
    credentialCache.Add(new Uri("http://domainORipaddress"), "NTLM", new NetworkCredential(
        ConfigurationManager.AppSettings["username"],
        ConfigurationManager.AppSettings["password"]
    ));

    Stream report = null;
    using (var httpClient = new HttpClient(new HttpClientHandler { Credentials = credentialCache }))
    {

        httpClient.Timeout = TimeSpan.FromMilliseconds(Timeout.Infinite);
        report = await httpClient.GetStreamAsync("reportUrl");
    }


    var contentDisposition = new ContentDisposition
    {
        FileName = "Report.pdf",
        Inline = false
    };
    Response.AppendHeader("Content-Disposition", contentDisposition.ToString());
    return File(report, "application/pdf");    
    
    //or use
    //Response.AppendHeader("Content-Disposition", String.Format("attachment;filename=\"{0}\"", reportName));
    //return File(reportPath, MediaTypeNames.Application.Pdf);
}