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
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 = '')
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