Open PDF in browser, not "Save As"

My users want an option to open stored PDFs in the browser window instead of being prompted to download the file.

Current code:

if(file_exists($file))
		{
			header('Content-Description: File Transfer');
			header('Content-Type: application/octet-stream');
			header('Content-Disposition: attachment; filename='.basename($file));
			header('Content-Transfer-Encoding: binary');
			header('Expires: 0');
			header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
			header('Pragma: public');
			header('Content-Length: ' . filesize($file));
			ob_clean();
			flush();
			readfile($file);
			exit;
		}

How would I write the headers to allow files to open in the browser window?