Wel pdf_open() could give a problem use pdf_new() and pdf_open_file() in combination with echo ""; do not forget to echo 'cause the pdf 'll stay empty in that case :)
axample:
<?php
$pdf = pdf_new();
pdf_open_file($pdf);
pdf_begin_page($pdf, 595, 842);
pdf_set_font($pdf, "Times-Roman", 30, "host");
pdf_set_value($pdf, "textrendering", 1);
pdf_show_xy($pdf, "A PDF document created in memory!", 50, 750);
pdf_end_page($pdf);
pdf_close($pdf);
$data = pdf_get_buffer($pdf);
header("Content-type: application/pdf");
header("Content-disposition: inline; filename=test.pdf");
header("Content-length: " . strlen($data));
echo $data;
?>