This is html part
<a href="{{route('download',$details->report_id)}}" type="button" class="btn btn-primary download" data-report_id="{{$details->report_id}}" >Download</a>
This is Route :
Route::get('/download/{id}', 'users\UserController@getDownload')->name('download')->middleware('auth');
This is function :
public function getDownload(Request $request,$id)
{
$file= public_path(). "/pdf/"; //path of your directory
$headers = array(
'Content-Type: application/pdf',
);
return Response::download($file.$pdfName, 'filename.pdf', $headers);
}