Sometime we need to send a invoice pdf in a invoice mail in magento, so here I am explaining how to add invoice pdf in invoice mail.

1) Override the app/code/core/Mage/Core/Model/Email/Template/Mailer.php file in your module

2) Add one protected variable $pdftemplate

3) Edit the send() function

4) at line around 70 put the following code

if ($this->pdftemplate)
      $pdftemplate = $this->variable;
else
      $pdftemplate = Mage::getModel('core/email_template');

5) At the end of the class put

public function addAttachment(Zend_Pdf $pdf, $filename){
    $file = $pdf->render();
            $this->pdftemplate = Mage::getModel('core/email_template');
            $attachment = $this->pdftemplate->getMail()->createAttachment($file);
    $attachment->type = 'application/pdf';
    $attachment->filename = $filename;
}

6) Override app/code/core/Mage/Sales/Model/Order/Invoice.php

7) Override the function sendEmail()

after 
        $mailer = Mage::getModel('core/email_template_mailer');

put

$pdf = Mage::getModel('sales/order_pdf_invoice')->getPdf(array($this));
$mailer->addAttachment($pdf,'invoice.pdf');

That's all you have to do.