How do I force-download different .pdfs in same script?

I have a webpage with 3 links.

When someone clicks on the link it redirected them to redirect.php and force-downloads the respective .pdf. Why is it that, all 3 links force-download the same .pdf file? How do I make it so each link downloads a different .pdf file?

You could read out the value of the paramter of the _GET Variable "dl" with $_GET['dl'].

<?php
if (isset($_GET['dl'])) {
  switch ($_GET['dl']) {
    // whitelist of downloadable files
    case  "mypdf1.pdf": 
    case  "mypdf2.pdf": 
    case  "mypdf3.pdf": 
      $file = basename($_GET['dl']);
    default: 
      die("no such file");
  }
} else {
  die("bad request");
}

// provide file  
header('Cache-control: private'); 
header('Cache-Control: no-store, no-cache, must-revalidate'); 
header('Cache-Control: pre-check=0, post-check=0, max-age=0'); 
header('Pragma: anytextexeptno-cache', true); 
header('Content-type: application/pdf'); 
header("Content-Disposition: attachment; filename=$file"); //external 
header('Last-Modified: '.gmdate('D, d M Y H:i:s') . ' GMT'); 
header('Expires: 0');
readfile("hiddenfiles/$file");    

HTML

  <a href="redirect.php?dl=mypdf1.pdf" target='_blank'>mypdf1.pdf</a>
  <a href="redirect.php?dl=mypdf2.pdf" target='_blank'>mypdf2.pdf</a>
  ...

If you want all 3 at once, than you could provide an archive like zip or tar.