pdf download wont work

This was already mentioned under c4 addins->pdf customized and no solution was offered.
Chrome and Firefox can download pdfs (generated from adobe and others) to my download folder but both refuse to display in the browser,
saying the file cant be downloaded. The files display perfectly using ci3 with setheaders and force_download.
I've tried all combinations of the code below and failed. Can anybody help? The project has come to a halt because I cant print.

Code:

return $this->response
        //->setHeader('Pragma','public')
        //->setHeader('Expires','0')
        //->setHeader('Cache-Control','must-revalidate, post-check=0, pre-check=0')
        //->setHeader('Content-Disposition','inline; filename="'.$filename.'"')
        //->setContentType('application/pdf')
        //->setHeader('Content-Length',filesize($fullname))
        ->download($fullname);//'',true);//,$data);

Many thanks, Bill