Basically, need to add Business number somewhere on the .pdf generated invoice in Magento.
It could be anywhere, top or bottom of the page.
You can customize the invoice layout by modifying class Mage_Sales_Model_Order_Pdf_Invoice, which can be found at:app/code/core/Mage/Sales/Model/Order/Pdf/Invoice.php
You have to modify getPdf() function which renders complete invoice PDF, somewhere in foreach loop. That should be something like:
$page->drawText('Business Number: ' . $businessNumber, 35, 35, 'UTF-8');
Also, you can modify color/font by using:
$page->setFillColor(new Zend_Pdf_Color_GrayScale(1)); $this->_setFontRegular($page, 10);
But, the main function for adding data to PDF is drawText(), and it's easy to use because you have only to provide text, x and y coordinates. If you're not sure what coordinates to put there, you can just try couple of times until you find position you want.
public function drawText($text, $x, $y, $charEncoding = '')