尝试生成pdf文件时出现FPDF问题

I have a button that must generate a pdf file of a table already filtered. I handle the click action with JavaScript here's the code.

$('#Exportar').on('click', function(){
 var oTable = $("#example").dataTable();
 var data = oTable._('tr', {"filter":"applied"});
 var ajaxurl = 'Exportar.php';

    $.ajax({ url: ajaxurl,
         data: {datosTabla: 'data'},
         type: 'post',
         success: function(output) {
                      // alert(output);
                  }
    });
})

My PHP file (Exportar.php) looks like this:

<?php
require('/fpdf/fpdf.php');

if(isset($_POST['datosTabla']) && !empty($_POST['datosTabla'])) {
$action = $_POST['datosTabla']; 

    $pdf = new FPDF();
    $pdf ->AddPage();
    $pdf ->SetFont('Arial','',10);
    $pdf ->Image('images/pdf_hover.png', 10, 10, 10, 10, 'png');
    $pdf->Cell(18,10,'',0);
    $pdf->Cell(150,10,'PDF exportado',1);
    $pdf ->SetFont('Arial','',9);
    $pdf->Cell(50,10,'Fecha: '.date('dd.-mm-Y'),'',0);

    $pdf->Ln(15);

    $pdf ->SetFont('Arial','B',11);
    $pdf->Cell(70, 8,'',0);
    $pdf->Cell(100, 8,'Reporte de Carteras',0);

    $pdf->Ln(23);

    $pdf ->SetFont('Arial','B',8);
    $pdf->Cell(15, 8,'columna1',0);
    $pdf->Cell(15, 8,'columna2',0);
    $pdf->Cell(15, 8,'columna3',0);

    $pdf ->SetFont('Arial','',11);
    $pdf->Output();
}
?>

I have examine the post with the dev tools from Chrome and the post it's been done correctly. I just do not know why it's not working. I have tested the functionality with a simple alert on the client side and works perfectly fine(comes out from the success handler).

My FPDF folder its at the same level as Exportar.php file. Can anyone lend me a hand on this? I have already tried checking other ppl posts and the faq from FPDF but it hasn't been much of a help.

On the other hand I have tried the example file that comes with FPDF and when I click the demo link the PDF is not been generated. I don't know why is this.

Thanks in advance for your help.