Creating FPDF directly in the browser using template_rediect

Use ob_start();, ob_clean(); before outputting the PDF to make any uwanted content is not written while cretaing PDF.

add_action("template_redirect", "checkRedirect");

function checkRedirect() {
    $url = (isset($_SERVER['HTTPS']) && $_SERVER['HTTPS'] === 'on' ? "https" : "http") . "://$_SERVER[HTTP_HOST]$_SERVER[REQUEST_URI]";
    $slug = strtolower(basename($url));

    if ($slug == "pdf") {
        ob_start();
        require_once(getPath() . "fpdf.php");
        $pdf = new \FPDF;
        $pdf->AddPage();
        $pdf->SetFont('Arial','B',16);
        $pdf->Cell(40,10,'Hello World!');
        ob_clean();
        $pdf->Output(); 
        exit; // this better then die
    } 
}