写一段在Android app内的html页面里下载pdf文件到本地的代码

以下是一个简单的在Android app内的html页面里下载pdf文件到本地的代码示例：

在html页面中添加一个下载链接：

<a href="pdf/sample.pdf" download>Download PDF</a>

在 Android 的 WebView 中启用文件下载：

WebView webView = findViewById(R.id.webView);
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.setMimeType(mimetype);
        String cookies = CookieManager.getInstance().getCookie(url);
        request.addRequestHeader("cookie", cookies);
        request.addRequestHeader("User-Agent", userAgent);
        request.setDescription("Downloading file...");
        request.setTitle(URLUtil.guessFileName(url, contentDisposition, mimetype));
        request.allowScanningByMediaScanner();
        request.setNotificationVisibility(DownloadManager.Request.VISIBILITY_VISIBLE_NOTIFY_COMPLETED);
        request.setDestinationInExternalPublicDir(Environment.DIRECTORY_DOWNLOADS, URLUtil.guessFileName(url, contentDisposition, mimetype));
        DownloadManager dm = (DownloadManager) getSystemService(DOWNLOAD_SERVICE);
        dm.enqueue(request);
        Toast.makeText(getApplicationContext(), "Downloading File", Toast.LENGTH_LONG).show();
    }
});

这段代码会在 WebView 上设置一个 DownloadListener，当用户点击下载链接时，会启动一个下载任务，并将文件保存到 Android 设备的 Downloads 文件夹中。