main part of a Hikashop plugin for branding sold PDFs

<?php
/**
 * @package     HikaShop for Joomla!
 * @version     1.5.5
 */
defined('_JEXEC') or die('Restricted access');
?>
<?php
class plgHikashopBranding extends JPlugin
{
    function onBeforeDownloadFile(&$file,&$do) {
        try {
            if (substr($file, strlen($file) - 4)  === ".pdf") {
                $user =& JFactory::getUser();
                $username = $user->username;
                $realname = $user->name;
                $fontSize = 12;
                $yTextPos = 10;
                $branding = "Licensed for: " . $realname;

                define('DS', DIRECTORY_SEPARATOR);
                define('JPATH_BASE', dirname(__FILE__) . DS . '..' . DS . '..' . DS . '..'); // assuming we need to go up 3 steps to get to the Joomla root
                set_include_path(dirname(__FILE__)); // set include_path for external library Zend Framework
                require_once(JPATH_BASE . DS . 'plugins' .DS . 'hikashop' . DS . 'branding' . DS . 'Zend' .DS . 'Pdf.php');

                $pdf = Zend_Pdf::load($file);
                //$font = Zend_Pdf_Font::fontWithName(Zend_Pdf_Font::FONT_HELVETICA); // works without having a TTF font file put in place
                $font = Zend_Pdf_Font::fontWithPath(JPATH_BASE . DS . 'plugins' .DS . 'hikashop' . DS . 'branding' . DS . 'calibri.ttf');

                // calculate width of branding (needed for centered placement of the branding)
                $pattern_s = array(utf8_encode('ö'),utf8_encode('ü'),utf8_encode('ä'),
                                utf8_encode('Ö'),utf8_encode('Ü'),utf8_encode('Ä'),
                                utf8_encode('ß'));
                $replace_s = array(utf8_encode('o'),utf8_encode('u'),utf8_encode('a'),
                                utf8_encode('O'),utf8_encode('U'),utf8_encode('A'),
                                utf8_encode('S'));
                $string = $branding;
                $string = str_replace($pattern_s,$replace_s,$string);
                $drawingString = iconv('', 'UTF-16BE', $string);
                $characters = array();
                for ($i = 0; $i < strlen($drawingString); $i++) {
                    $characters[] = (ord($drawingString[$i++]) << 8) | ord($drawingString[$i]);
                }
                $glyphs = $font->glyphNumbersForCharacters($characters);
                $widths = $font->widthsForGlyphs($glyphs);
                $stringWidth = (array_sum($widths) / $font->getUnitsPerEm()) * $fontSize;

                // place branding centered on every page
                foreach ($pdf->pages as &$page) {
                    $page->setFont($font, $fontSize);
                    $xTextPos = ($page->getWidth() - $stringWidth) / 2;
                    $page->drawText($branding, $xTextPos, $yTextPos);
                }

                // save file with the username included in the filename
                $tempfile = substr($file, 0, strlen($file) - 4) . "_" . $username . ".pdf";
                $pdf->save($tempfile);
                $file = $tempfile;
            }
        }
        catch (Exception $e) {
            $do = false;
            $app =& JFactory::getApplication();
            $app->enqueueMessage(JText::_('Error when branding PDF: ' . $e->getMessage()));
        }
    }
}