<form action="soubor.php" target="foo" onSubmit="window.open('', 'foo', 'width=450,height=300,status=yes,resizable=yes,scrollbars=yes')">
Ještě bys měl u tagu form doplnit atribut method.
<form action="soubor.php" target="foo" onSubmit="window.open('', 'foo', 'width=450,height=300,status=yes,resizable=yes,scrollbars=yes')">