If you have the URL of PDF which is situated on some other domain. You can allow it to be downloaded via your laravel application with following code 

$pdf = file_get_contents('http://www.africau.edu/images/default/sample.pdf');
Storage::disk('local')->put('samplepdf.pdf', $pdf);
return response()->download(storage_path().'/app/samplepdf.pdf')->deleteFileAfterSend(true);
Get the file contents to your local server and save it as pdf via Storage facade. Return the download in the response and delete once downloaded. That's about it.