webview.setDownloadListener(new DownloadListener() {
    public void onDownloadStart(String url, String userAgent, String contentDisposition, String mimetype, long contentLength) {

        Uri webpage = Uri.parse(url);

        DownloadManager.Request request = new DownloadManager.Request(webpage);
        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.setDestinationInExternalFilesDir(MainActivity.this,Environment.DIRECTORY_DOWNLOADS,".pdf");
        DownloadManager dm = (DownloadManager) getSystemService(DOWNLOAD_SERVICE);
        dm.enqueue(request);
        
        Toast.makeText(getApplicationContext(), "Downloading File",Toast.LENGTH_LONG).show();
    }
});

package com.bilhos.mobilsandiktakip;

import androidx.appcompat.app.AppCompatActivity;
import androidx.core.app.ActivityCompat;
import androidx.core.content.ContextCompat;

import android.Manifest;
import android.app.DownloadManager;
import android.content.pm.PackageManager;
import android.net.Uri;
import android.os.Build;
import android.os.Bundle;
import android.os.Environment;
import android.webkit.CookieManager;
import android.webkit.DownloadListener;
import android.webkit.URLUtil;
import android.webkit.WebView;
import android.webkit.WebViewClient;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

private static final int PERMISSIONS_REQUEST_CODE = 100;
private static final String PERMISSIONS_STORAGE = Manifest.permission.WRITE_EXTERNAL_STORAGE;

private WebView webView;

@Override
    protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);

webView = findViewById(R.id.web);
webView.getSettings().setJavaScriptEnabled(true);
webView.setWebViewClient(new WebViewClient());
webView.loadUrl("https://bilhos.com.tr/");

// Check permission for storage
        if (ContextCompat.checkSelfPermission(this, PERMISSIONS_STORAGE) != PackageManager.PERMISSION_GRANTED) {
ActivityCompat.requestPermissions(this, new String[]{PERMISSIONS_STORAGE}, PERMISSIONS_REQUEST_CODE);
}

// Set download listener to enable downloading from 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);
//------------------------COOKIE!!------------------------
                String cookies = CookieManager.getInstance().getCookie(url);
request.addRequestHeader("cookie", cookies);
//------------------------COOKIE!!------------------------
                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();
}
});
}

// Handle permission request result
    @Override
    public void onRequestPermissionsResult(int requestCode, String[] permissions, int[] grantResults) {
super.onRequestPermissionsResult(requestCode, permissions, grantResults);
if (requestCode == PERMISSIONS_REQUEST_CODE) {
if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
// Permission granted
            } else {
// Permission denied
                Toast.makeText(this, "Permission denied. App cannot download files.", Toast.LENGTH_SHORT).show();
}
}
}
}