What are some common pitfalls when trying to send emails with PDF attachments using PHP?

// Set the content type and encoding for the attachment
$mime_boundary = "----=_NextPart_" . md5(uniqid(time()));
$headers = "MIME-Version: 1.0\r\n";
$headers .= "Content-Type: multipart/mixed; boundary=\"$mime_boundary\"\r\n";

// Create the attachment
$file = "path/to/file.pdf";
$attachment = chunk_split(base64_encode(file_get_contents($file)));

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

// Send the email with attachment
$to = "recipient@example.com";
$subject = "Email with PDF attachment";
$mail = mail($to, $subject, $message, $headers);

if($mail) {
    echo "Email with PDF attachment sent successfully.";
} else {
    echo "Failed to send email with PDF attachment.";
}