DownloadPdfFile Function With SystemNetHttp

private async Task <string> DownloadPdfFileAsync()
        {
            var filePath = Path.Combine(FileSystem .AppDataDirectory, "CifradoEnRedes_122017.pdf");
            if (File.Exists(filePath))
            {
                return filePath;
            }

            var httpClient = new HttpClient();
            var pdfBytes = await httpClient.GetByteArrayAsync("https://1drv.ms/b/s!AtMYJuNNyNicilx0lIqs4Za0F9UA?e=T8uc1V");
            try
            {
                File.WriteAllBytes(filePath, pdfBytes);

                return filePath;
            }
            catch (Exception ex)
            {
                await Application.Current.MainPage.DisplayAlert("Error", ex.Message, "Ok");
            }

            return null;
        }
private async  void Btn2Click(object sender, EventArgs e)
        {
            App.MasterDet.IsPresented =false;
            await App.MasterDet.Detail.Navigation.PushAsync(new DISPERSION());
            var filePath = await DownloadPdfFileAsync();

            if (filePath != null)
            {
                await Launcher.OpenAsync(new OpenFileRequest
                {
                    File = new ReadOnlyFile(filePath)
                });
            }

        }