在webView中下载带.pdf扩展名的文件

Question

我里面的本地html文件很少，资产文件夹包含可供下载给用户的pdf文件。但是我无法在webView中下载文件。我搜索整个谷歌和最好的我想出了

  webView.setDownloadListener(new DownloadListener() {

        @Override
        public void onDownloadStart(String url, String userAgent,
                                    String contentDisposition, String mimetype,
                                    long contentLength) {
            DownloadManager.Request request = new DownloadManager.Request(
                    Uri.parse(url));

            request.allowScanningByMediaScanner();
            request.setNotificationVisibility(DownloadManager.Request.VISIBILITY_VISIBLE_NOTIFY_COMPLETED); //Notify client once download is completed!
            request.setDestinationInExternalPublicDir(Environment.DIRECTORY_DOWNLOADS, "Name of your downloadble file goes here, example: Mathematics II ");
            DownloadManager dm = (DownloadManager) getSystemService(DOWNLOAD_SERVICE);
            dm.enqueue(request);
            Toast.makeText(getApplicationContext(), "Downloading File", //To notify the Client that the file is being downloaded
                    Toast.LENGTH_LONG).show();

        }
    });

但是，当我点击链接时，它显示Dowanloading，然后在Notification的面板上显示Can not Dowanload Error。

我的manifest.xml文件是

<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<uses-permission android:name="android.permission.ACCESS_WIFI_STATE" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

我正在使用模拟器Bluestack来测试我的应用程序。

非常感谢任何帮助。