Magento: Add Custom Text At the Bottom of Invoice Pdf

Copy the file app/code/core/Mage/Sales/Model/Order/Pdf/Invoice.php to app/code/local/Mage/Sales/Model/Order/Pdf/Invoice.php and add the below function at the end of the file.

public function insertConditions($page) {
    $page->drawLine(25, $this->y, 570, $this->y);
    $this->y -= 25;
    $page->drawText(Mage::helper('sales')->__('Your custom text here'), 35,  $this->y, 'UTF-8');
}

Now, we need to call this insertConditions() function in the getPdf() function like below:

public function getPdf($invoices = array())
    {
        $this->_beforeGetPdf();
        $this->_initRenderer('invoice');

        $pdf = new Zend_Pdf();
        $this->_setPdf($pdf);
        $style = new Zend_Pdf_Style();
        $this->_setFontBold($style, 10);

        foreach ($invoices as $invoice) {
            if ($invoice->getStoreId()) {
                Mage::app()->getLocale()->emulate($invoice->getStoreId());
                Mage::app()->setCurrentStore($invoice->getStoreId());
            }
            $page  = $this->newPage();
            $order = $invoice->getOrder();
            /* Add image */
            $this->insertLogo($page, $invoice->getStore());
            /* Add address */
            $this->insertAddress($page, $invoice->getStore());
            /* Add head */
            $this->insertOrder(
                $page,
                $order,
                Mage::getStoreConfigFlag(self::XML_PATH_SALES_PDF_INVOICE_PUT_ORDER_ID, $order->getStoreId())
            );
            /* Add document text and number */
            $this->insertDocumentNumber(
                $page,
                Mage::helper('sales')->__('Invoice # ') . $invoice->getIncrementId()
            );
            /* Add table */
            $this->_drawHeader($page);
            /* Add body */
            foreach ($invoice->getAllItems() as $item){
                if ($item->getOrderItem()->getParentItem()) {
                    continue;
                }
                /* Draw item */
                $this->_drawItem($item, $page, $order);
                $page = end($pdf->pages);
            }
            /* Add totals */
            $this->insertTotals($page, $invoice);
            if ($invoice->getStoreId()) {
                Mage::app()->getLocale()->revert();
            }
        }
        $this->insertConditions($page); // the custom text is added here
        $this->_afterGetPdf();
        return $pdf;
    }

Here, we rewrite the core files instead of changing them directly.