$filename= "{$membership->id}.pdf"; 
$filelocation = "D:\\wamp\\www\\project\\custom";//windows
$filelocation = "/var/www/project/custom"; //Linux

$fileNL = $filelocation."\\".$filename;//Windows
$fileNL = $filelocation."/".$filename; //Linux

$this->pdf->Output($fileNL, 'F');