require_once 'Zend/Pdf.php';

$pdf = Zend_Pdf::load('blank.pdf');

$page = $pdf->pages[0];

$font = Zend_Pdf_Font::fontWithName(Zend_Pdf_Font::FONT_HELVETICA);

$page->setFont($font, 12);

$page->drawText('Hello world!', 72, 720);

$pdf->save('zend.pdf');