Here is a final working copy that won't freak out Microsoft Explorer if you are using sessions. Thanks to everyone else who came before. This is not as simple as I thought it would be.

the user would pass a call to the page:
http://mysite/getfile.php?file=products.pdf

include 'base.inc'; // inlcude base code, start session  and manage users

// This loads the file global from the post/get variables
// For security reasons register globals is disabled
LoadPostGet('file');

$filename = '/data/files/' . $file;
if(file_exists($filename)){
  $FILECMD = '/usr/bin/file';
  $contentType = '';
  $fp=popen("$FILECMD -bin $filename", 'r');
  if (!$fp) $contentType='application/octet-stream';
  else {
    while($string=fgets($fp, 1024)) $contentType .= $string;
    pclose($fp);
  }
  if(strpos($HTTP_SERVER_VARS['HTTP_USER_AGENT'], 'MSIE')){
    // IE cannot download from sessions without a cache
    header('Cache-Control: public');
  }
  header("Content-type: $contentType");
  header("Content-Disposition:inline; filename=\"".$file."\"");
  header("Content-length:".(string)(filesize($filename)));
  $fd=fopen($filename,'rb');
  while(!feof($fd)) {
    print fread($fd, 4096);
  }
  fclose($fd);
}else{
  print "File Not Found";
}