Pdf Join Cat wrong

Pdf Join Cat wrong

shell_exec('pdftk offerletter/'.$_GET['id'].'.pdf LAMPIRAN_A.pdf cat
output offerletter2/'.$_GET['id'].'.pdf dont_ask');
header('location:offerletter2/'.$_GET['id'].'.pdf');
situation : example : $_GET['id']=123456789;
student will download offerletter name 123456789, which it will save under
folder offerletter. And in offerletter folder, have LAMPIRAN_A.pdf that
will join with the student offerletter, and after join, it will output to
another folder, which name offerletter2
error : cannot find offerletter2/123456789.pdf