Mine

$headers = "From: orderdesk <info@guardianchem.ca>";
 // Generate a boundary string 
 $semi_rand = md5(time()); 
 $mime_boundary = "==Multipart_Boundary_x{$semi_rand}x"; 
  
 // Add the headers for a file attachment 
 $headers .= "\nMIME-Version: 1.0\n" . 
             "Content-Type: multipart/mixed;\n" . 
             " boundary=\"{$mime_boundary}\"";
$subject = "Incoming Order";
$message = "Please find attached, completed order form";
 // Add a multipart boundary above the plain message 
 $message = "This is a multi-part message in MIME format.\n\n" . 
            "--{$mime_boundary}\n" . 
            "Content-Type: text/plain; charset=\"iso-8859-1\"\n" . 
            "Content-Transfer-Encoding: 7bit\n\n" . 
            $message . "\n\n";
$to=$_POST["From"].",info@guardianchem.ca";
 // Base64 encode the file data 
 $contents = chunk_split(base64_encode($contents));

 // Add file attachment to the message 
 $message .= "--{$mime_boundary}\n" . 
             "Content-Type: application/pdf;\n" . 
             " name=\"order.pdf\"\n" . 
             "Content-Disposition: attachment;\n" . 
             " filename=\"order.pdf\"\n" . 
             "Content-Transfer-Encoding: base64\n\n" . 
             $contents . "\n\n" . 
             "--{$mime_boundary}--\n"; 

// Send the message 
$ok = @mail($to, $subject, $message, $headers);