If you are confused and trying to figure out how to just simply send a mail with and attachment using drupal_mail(), with a given $body and $subject, this below code may help you.

/*hook_mail($key, &$message, $params);*/
//for detail information please visit https://api.drupal.org/api/drupal/modules!system!system.api.php/function/hook_mail/7


<?php
/**
Send Email with an attachment
*/

function custom_mail($key, &$message, $params) {
  if (isset($params['subject'])) {
    $message['subject'] = $params['subject'];
  } 
  if (isset($params['body'])) {
    $message['body'][] = $params['body']; 
  }
  if (isset($params['headers']) && is_array($params['headers'])) {   
    $message['headers'] += $params['headers'];
  }

  if (isset($params['attachment'])) {
    $message['params']['attachments'][] = $params['attachment'];
  }
  $message['headers']['Reply-To'] = 'abc@yahoo.in';
}

  $from = 'NJDP Error Log File';
  $body = 'hello World!';
  $subject = 'PHP Error Report Mail with Attachment';
  $filepath = DRUPAL_ROOT.'/php_errors.log';
  $headers = array(
                    'MIME-Version' => '1.0',
                    'Content-Type' => 'text/html; charset=UTF-8;',
                    'Content-Transfer-Encoding' => '8Bit',
                    'X-Mailer' => 'Drupal'
                );
                drupal_set_message($filepath);
    $attachment = array(
      'filecontent' => file_get_contents($filepath),
      'filename' => 'php_errors.log',
      'filemime' => 'application/pdf',
    );
  $message = drupal_mail('custom', 'register_mail', $to, user_preferred_language($user), array('body' => $body, 'subject' => $subject, 'attachment' => $attachment), $from, TRUE);