Applying watermarks on pdf files when users try to download the files

Although there are several very good PDF libs for PHP, if I were writing such a program I'd just shell out to run pdftk but you' still need to generate your watermark.

$tempfile=tempnam();
system("pdftk input_file.pdf background watermark.pdf output $tempfile dont_ask", $errcode);
if (!$errcode && $ih=fopen($tempfile, 'r')) {
    header('Content-Type: application/pdf');
    fpassthru($ih);
    fclose($ih);
} else {
    print "Whoops";
}
unlink($tempfile);

Needed to do this yesterday and here's how without any external non-PHP libs that need to be installed. The only 2 libs needed are both PHP libs and are easy to get.

• FPDF -> http://www.fpdf.org/ latest version
• FPDI -> http://www.setasign.de/products/pdf-php-solutions/fpdi/downloads/ (make sure to throw the fpdf_tpl.php file in the same folder as fpdi.php)

Now you can use the class below to achieve watermarking

/** MAKE SURE TO HAVE THE INCLUDES RUNNING PROPERLY */
require_once('FPDF/fpdf.php');
require_once('FPDI/fpdi.php');

class WaterMark
{
    public $pdf, $file, $newFile,
        $wmText = "STACKOVERFLOW";

    /** $file and $newFile have to include the full path. */
    public function __construct($file, $newFile)
    {
        $this->pdf =& new FPDI();
        $this->file = $file;
        $this->newFile = $newFile;
    }

    /** $file and $newFile have to include the full path. */
    public static function applyAndSpit($file, $newFile)
    {
        $wm = new WaterMark($file, $newFile);

        if($wm->isWaterMarked())
            return $wm->spitWaterMarked();
        else{
            $wm->doWaterMark();
            return $wm->spitWaterMarked();
        }
    }

    /** @todo Make the text nicer and add to all pages */
    public function doWaterMark()
    {
        $currentFile = $this->file;
        $newFile = $this->newFile;

        $pagecount = $this->pdf->setSourceFile($currentFile);

        for($i = 1; $i <= $pagecount; $i++){
                            $this->pdf->addPage();
            $tplidx = $this->pdf->importPage($i);
            $this->pdf->useTemplate($tplidx, 10, 10, 100);
            // now write some text above the imported page
            $this->pdf->SetFont('Arial', 'I', 40);
            $this->pdf->SetTextColor(255,0,0);
            $this->pdf->SetXY(25, 135);
            $this->_rotate(55);
            $this->pdf->Write(0, $this->wmText);
                            $this->_rotate(0);
        }

        $this->pdf->Output($newFile, 'F');
    }

    public function isWaterMarked()
    {
        return (file_exists($this->newFile));
    }

    public function spitWaterMarked()
    {
        return readfile($this->newFile);
    }

    protected function _rotate($angle,$x=-1,$y=-1) {

        if($x==-1)
            $x=$this->pdf->x;
        if($y==-1)
            $y=$this->pdf->y;
        if($this->pdf->angle!=0)
            $this->pdf->_out('Q');
        $this->pdf->angle=$angle;

        if($angle!=0){
            $angle*=M_PI/180;
            $c=cos($angle);
            $s=sin($angle);
            $cx=$x*$this->pdf->k;
            $cy=($this->pdf->h-$y)*$this->pdf->k;

            $this->pdf->_out(sprintf(
                'q %.5f %.5f %.5f %.5f %.2f %.2f cm 1 0 0 1 %.2f %.2f cm',
                $c,$s,-$s,$c,$cx,$cy,-$cx,-$cy));
        }
    } 

}

You now can run this as:

WaterMark::applyAndSpit($fileWithFullPath);

There is an excellent opensource php library http://www.tcpdf.org/ , I use it for all pdf generating tasks.