PHP PDF attachment doesn't open in outlook

I have a code to generate a PDF and then attach it in a PHP mail script.
The PDf comes through just fine when I open gmail in my browser.

If I open my gmail account through outlook, I see the little attachment symbol, but the email comes through totally blank and I can't access the PDF.
Is there something else I am supposed to change?


    $name        = "Devorah";
    $email       = "ffrid@****";
    $to          = "$name <$email>";
    $from        = "info@*****";
    $subject     = "Application information";
    $mainMessage = "Hi, here's the file.";
    $fileatt     = "application.pdf";
    $fileatttype = "application/pdf";
    $fileattname =  $_POST['lastName'] . "_" . $id .".pdf";
    $headers = "From: $from";

    // File
    $file = fopen($fileatt, 'rb');
    $data = fread($file, filesize($fileatt));
    fclose($file);
   
    // This attaches the file
    $semi_rand     = md5(time());
    $mime_boundary = "==Multipart_Boundary_x{$semi_rand}x";
    $headers      .= "\nMIME-Version: 1.0\n" .
    "Content-Type: multipart/mixed;\n" .
    " boundary=\"{$mime_boundary}\"";
    $message = "This is a multi-part message in MIME format.\n\n" .
    "-{$mime_boundary}\n" .
    "Content-Type: text/plain; charset=\"iso-8859-1\n" .
    "Content-Transfer-Encoding: 7bit\n\n" .
    $mainMessage  . "\n\n";
       
    $data = chunk_split(base64_encode($data));
    $message .= "--{$mime_boundary}\n" .
    "Content-Type: {$fileatttype};\n" .
    " name=\"{$fileattname}\"\n" .
    "Content-Disposition: attachment;\n" .
    " filename=\"{$fileattname}\"\n" .
    "Content-Transfer-Encoding: base64\n\n" .
    $data . "\n\n" .
    "-{$mime_boundary}-\n";
   
    // Send the email
    mail($to, $subject, $message, $headers);