Private Sub LoadDocument()
        ' Web からファイルを読み込みます
        Dim client As New HttpClient()
        Dim url As String = "http://cdn.componentone.com/files/win8/Win8_UXG_RTM.pdf"
        Try
              Dim stream = Await client.GetStreamAsync(New Uri(url, UriKind.Absolute))
            pdfViewer.LoadDocument(stream)
        Catch
            Dim dialog = New MessageDialog("There was an error attempting to download the document.")
            dialog.ShowAsync()     
        End Try
    End Sub

private async void LoadDocument()
{  
  // Web からファイルを読み込みます
    HttpClient client = new HttpClient();
    string url = “http://cdn.componentone.com/files/win8/Win8_UXG_RTM.pdf”;
    try
    {
        var stream = await client.GetStreamAsync(new Uri(url, UriKind.Absolute));    
        pdfViewer.LoadDocument(stream);
    }
    catch 
{ 
    var dialog = new MessageDialog("There was an error attempting to download the document.");
        dialog.ShowAsync();
    }
}