Looks to me that you're calling your download manager in the wrong place.
Intent intent = new Intent();
intent = getIntent();
contactUri = intent.getData();
Uri src_uri = Uri.parse(contactUri.getPath());
Uri dst_uri = Uri.parse("file:///mnt/sdcard/download/testing.pdf");
DownloadManager.Request req = new DownloadManager.Request(src_uri);
req.setDestinationUri(dst_uri);
DownloadManager dm = (DownloadManager) getSystemService(DOWNLOAD_SERVICE);
dm.enqueue(req);
try this:
Intent intent = new Intent();
intent = getIntent();
contactUri = intent.getData();
Uri src_uri = Uri.parse(contactUri.getPath());
Uri dst_uri = Uri.parse("file:///mnt/sdcard/download/testing.pdf");
DownloadManager dm = (DownloadManager) getSystemService(DOWNLOAD_SERVICE);
DownloadManager.Request req = new DownloadManager.Request(src_uri);
req.setDestinationUri(dst_uri);
dm.enqueue(req);