FPDI watermark on pdf

I have been trying to add a watermark on an existing pdf, and so far I have done it successfully but the Issue that I am facing is that a watermark is being added over the content, if anyone knows how to set it behind content, I would be really grateful for any help. 
Screenshot of  generated pdf
here's my code - 

PHP Code:

function fpdi()
    {
        $file = 'dynamics/client-documents/P8/C10/1670573576_e86485fff7bd3890261c.pdf';

        $pdf = new Fpdi();
        $watermarkText = setting(UPLOAD_SETTING.'watermarkText');

        $pages_count = $pdf->setSourceFile($file);
        for ($i = 1; $i <= $pages_count; $i ++) {
            $pdf->AddPage();
            $tplIdx = $pdf->importPage($i);
            $pdf->useTemplate($tplIdx, 0, 0);
            $pdf->SetFont('Arial','B',50);
            $pdf->SetTextColor(192, 192, 192);
            $this->addWatermark(105, 220, $watermarkText, 45, $pdf);
            $pdf->SetXY(25, 25);
        }
        $pdf->Output('D', 'my-document.pdf');
    }



    function addWatermark($x, $y, $watermarkText, $angle, $pdf)
    {
        $angle = $angle * M_PI / 180;
        $c = cos($angle);
        $s = sin($angle);
        $cx = $x * 1;
        $cy = (300 - $y) * 1;
        $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));
        $pdf->Text($x, $y, $watermarkText);
        $pdf->_out('Q');
    } 


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