为什么较大的 pdf 文件会导致从 android WebViewClient 下载的格式无效？

Question

我可以在 WebView 中下载更小的 pdf 文件。 但大尺寸文件会报错 Invalid Format.Moreover 下载的文件大小为 2B。 任何帮助，将不胜感激。

 mWebView.setDownloadListener(new DownloadListener() {
        @Override
        public void onDownloadStart(String url, String userAgent,
                                    String contentDisposition, String mimetype,
                                    long contentLength) {
            if (ContextCompat.checkSelfPermission(MainActivity.this,
                    Manifest.permission.WRITE_EXTERNAL_STORAGE)
                    != PackageManager.PERMISSION_GRANTED) {
                ActivityCompat.requestPermissions(MainActivity.this,
                        new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE},
                        PERMISSION_READ_PHONE_STATE);
                Toast.makeText(getApplicationContext(), "Please retry.",
                        Toast.LENGTH_LONG).show();
            } else {
                String filename = URLUtil.guessFileName(url, contentDisposition, mimetype);
                DownloadManager.Request request = new DownloadManager.Request(
                        Uri.parse(url));
                String cookies = CookieManager.getInstance().getCookie(url);
                request.addRequestHeader("cookie", cookies);
                request.setDescription("File Download");
                request.addRequestHeader("User-Agent", userAgent);
                request.setTitle(filename);
                request.setMimeType(mimetype);
                request.allowScanningByMediaScanner();
                request.setNotificationVisibility(DownloadManager.Request.VISIBILITY_VISIBLE_NOTIFY_COMPLETED); //Notify client once download is completed!
                request.setDestinationInExternalPublicDir(Environment.DIRECTORY_DOWNLOADS, filename);
                DownloadManager dm = (DownloadManager) getSystemService(DOWNLOAD_SERVICE);
                dm.enqueue(request);
                spinner.setVisibility(View.VISIBLE);
                Toast.makeText(getApplicationContext(), "Downloading File...",
                        Toast.LENGTH_LONG).show();
            }
        }
    });