FPDF for creating pdf diplomas

Before anything else, I must say I am new on building WordPress functions. I have a website with online courses. Students have to sit exams and at the end they get a score, an average of all the exams. I am trying to build a simple dynamic PDF diploma with their name and average score, no more, which I will download by clicking on a button from a table/list. I found the class FPDF, unzipped it and uploaded it through FTP. Then, I made an include( 'fpdf183/fpdf.php'); on my functions.php in order to retrieve the class FPDF. After that, I went to my student-page.php and wrote this code to create the PDF:

 $sql =SELECT DISTINCT(ID),`user_registered`...();
    
        $request = $wpdb-get_results($sql); ?
        table id=example class=display cell-border compact stripe style=width:100%
          thead
            tr
                thName/th
                thAverage score/th
                thDownload Diploma/th
            /tr
          /thead
          tbody
    
      foreach ($request as $value) {

        $pdf = new FPDF();
        if( isset($_POST['generate_pdf'])){
    
          global $pdf;
          $title_line_height = 10;
          $content_line_height = 8;        
    
          $pdf-AddPage();
          $pdf-SetFont( 'Arial', '', 42 );
          $pdf-Write(20, 'This is a sample');
          $pdf-Write(20, $value-name);
          $pdf-Write(20, $value-average);
          $pdf-Output('D','sample.pdf');
         }
         tr
          td?php echo $value-name ?/td
          td?php echo $value-average ?/td
          td
            form method=post id=as-fdpf-form
              button class=button button-primary type=submit name=generate_pdf value=generateDownload/button
            /form
          /td
         /tr

        /tbody
       /table

I didn't copy the whole code, just the most important so that you get the idea. I do not know what I am doing wrong (maybe everything), but I would appreciate your help. :)