<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <preference for="MagentoSalesModelOrderPdfInvoice" type="[VendorName][ModuleName]ModelOrderPdfInvoice" />
</config>

/**
 * Set font as regular
 *
 * @param  Zend_Pdf_Page $object
 * @param  int $size
 * @return Zend_Pdf_Resource_Font
 */
protected function _setFontRegular($object, $size = 7)
{
    $font = Zend_Pdf_Font::fontWithPath(
        $this->_rootDirectory->getAbsolutePath('lib/internal/LinLibertineFont/LinLibertine_Re-4.4.1.ttf')
    );
    $object->setFont($font, $size);
    return $font;
}

/**
 * Set font as bold
 *
 * @param  Zend_Pdf_Page $object
 * @param  int $size
 * @return Zend_Pdf_Resource_Font
 */
protected function _setFontBold($object, $size = 7)
{
    $font = Zend_Pdf_Font::fontWithPath(
        $this->_rootDirectory->getAbsolutePath('lib/internal/LinLibertineFont/LinLibertine_Bd-2.8.1.ttf')
    );
    $object->setFont($font, $size);
    return $font;
}

/**
 * Set font as italic
 *
 * @param  Zend_Pdf_Page $object
 * @param  int $size
 * @return Zend_Pdf_Resource_Font
 */
protected function _setFontItalic($object, $size = 7)
{
    $font = Zend_Pdf_Font::fontWithPath(
        $this->_rootDirectory->getAbsolutePath('lib/internal/LinLibertineFont/LinLibertine_It-2.8.2.ttf')
    );
    $object->setFont($font, $size);
    return $font;
}