TCPDF Save file to folder?

try this

$pdf->Output('kuitti'.$ordernumber.'.pdf', 'F');

$pdf->Output() takes a second parameter $dest, which accepts a single character. The default, $dest='I' opens the PDF in the browser.

Use F to save to file

$pdf->Output('/path/to/file.pdf', 'F')

this stores the generated pdf file in your custom folder of your project

$filename= "{$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');