PrestaShop/PrestaShop — pull request #1062 by codeurWeb (2013-11-30 18:49:03+00:00)
Title: [-] PDF : Save file when display is off in PDFGenerator.php

Change S to F when $display is false. Else, the F option is not settable.
Documentation:
S: return the document as a string (name is ignored).
F: save to a local server file with the name given by name.

Other solution is add :
elseif ($display == 'F')
$output = 'F';