TCPDF does not generate PDF file under ajax call

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Javascript

HTML

PHP

Ajax

I am trying to generate and download pdf file using the library TCPDF of php. I pass the parameters to php thru ajax using javascript: These parameters comes from html table, that I transfer to the PHP as an array :Here is the portion of my Javascript code:

var myTableArray = [];
    var nb = 0;

$("table#Table tr").each(function() {
    var arrayOfThisRow = [];
    var tableData = $(this).find('td');
    if (tableData.length > 0) {
        tableData.each(function() { arrayOfThisRow.push($(this).text()); });
        myTableArray.push(arrayOfThisRow);
        nb = nb +1;
    }
});

    jQuery.ajax({  
         type: "post",  
         url: "Generate_PDF.php",  
         data: {table:myTableArray, Line :nb },
         success: function(count){  
             alert("done"); 
         }  
    }); 

And here is the PHP Code:

function fetch_data()  
 {
$aDataTableDetailHTML = ' ';
     $count = $_POST['Line'];
      $line = 0;
      $row = 0;
      for($line; $line < $count; $line++)
      {
          for ($row =0; $row < 2; $row++)
          {
             If ($row == 0)
                {
    $aDataTableDetailHTML .= '<tr><td>'.$_POST['table'][$line][$row].'</td>';
                } enter code here
                else
                {
    $aDataTableDetailHTML .= '<td>'.$_POST['table'][$line][$row].'</td></tr>'; 
                }
          }
      }
      $aDataTableDetailHTML1 = '<tr><td> introduction 123 </td><td> temps zero</td></tr>';
    return $aDataTableDetailHTML;

in the chrome consol ->Network->preiew I see that the pdf file does not get generated, and I see a strange long text that starts with:

%PDF-1.7
%âãÏÓ
7 0 obj
<< /Type /Page /Parent 1 0 R /LastModified (D:20170513141422+02'00') /Resources 2 0 R

This strange text is too long and it ends with:

0000006871 00000 n 
trailer
<< /Size 12 /Root 11 0 R /Info 9 0 R /ID [ <f7adf2ceab2607d2e2cecc277437d8cd> <f7adf2ceab2607d2e2cecc277437d8cd> ] >>
startxref
7080
%%EOF

Any Idea folks...!? Thanks

What I have tried:

I have Tried all I could...please see the explanation in my question, I feel free If you need any more clarifications.