mpdf is returning me a string instead of generating the pdf

Question

I’m using the library mpdf in my project, however at the time of generating the document it is returning me a string and is not generating anything, you know what can be?

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Google Chrome Developer Tools - Response:

Controller class:

public function get_relatorio_pdf(){

    $request_body = file_get_contents('php://input');
    $data = json_decode($request_body);

    if (!is_logged_in() || !$this->session->has_userdata('admin')){
        $this->load->view('login_view');
        return;
    }

    $this->load->library('pdf');//carrega a biblioteca mPDF

    $body = $this->pdf->get_relatorio_teste();

    $mpdf = $this->pdf->load();//cria um objeto de 'pdf'
    $mpdf->WriteHTML($body);
    $mpdf->Output('relatorio.pdf', 'I');//mostra o pdf no navegador
    return;
}

PDF.php

 public function load($param=NULL){

        if (file_exists('/xampp/htdocs/teste/vendor/autoload.php')) {//caminho no windows de desenvolvimento local
            include_once '/xampp/htdocs/teste/vendor/autoload.php';
            include_once '/xampp/htdocs/teste/vendor/mpdf/mpdf/mpdf.php';
        }else if(file_exists('/var/www/html/teste/vendor/autoload.php')){//caminho no ubuntu de desenvolvimento local
            include_once '/var/www/html/teste/vendor/autoload.php';
            include_once '/var/www/html/teste/vendor/mpdf/mpdf/mpdf.php';
        }else{//colocar caminho dos includes do servidor de producao da dirppg aqui
           /*include_once '/var/www/????/vendor/autoload.php'; //descobrir o caminho de autoload.php
            include_once '/var/www/????/vendor/mpdf/mpdf/mpdf.php';//descobrir o caminho de mpdf.php
           */
           var_dump("Erro");
        }


        if ($param == NULL) {
            $param = '"en-GB-x","A4","","",10,10,10,10,6,3';
        }

        $mpdf = new mPDF($param);


        return $mpdf;

    }

public function get_relatorio_teste(){
    $html = '<div>Olá mundo</div>';

    return $html;
}