Create OCMOD like the one shown below.

<?xml version="1.0" encoding="utf-8"?>
<modification>
    <name>PDF INVOICE Customization</name>
    <version>2.1.4</version>
    <author>HuntBee OpenCart Services</author>
    <link>https://www.huntbee.com</link>
    <code>huntbee_pdf_invoice_custom</code>

    <file path="admin/controller/extension/hbapps/hb_pdfinvoice.php">
	<operation>
            <search><![CDATA[array('image','model','sku','upc','ean','jan','isbn','mpn','weight','dimension','quantity','item_price','item_tax','price')]]></search>
            <add position="replace"><![CDATA[array('image','model','sku','act_size','upc','ean','jan','isbn','mpn','weight','dimension','quantity','item_price','item_tax','price')]]></add>
        </operation>
  </file>
  
  <file path="catalog/model/extension/module/hb_pdfinvoice.php">
	<operation>
            <search><![CDATA[$productinfo['sku'],]]></search>
            <add position="after"><![CDATA['act_size'    	=> $productinfo['act_size'],]]></add>
        </operation>

        <operation>
            <search><![CDATA[array('model','sku','upc','ean','jan','isbn','mpn','weight','dimension','quantity','item_price','item_tax','price')]]></search>
            <add position="replace"><![CDATA[array('model','sku','act_size','upc','ean','jan','isbn','mpn','weight','dimension','quantity','item_price','item_tax','price')]]></add>
        </operation>

	<operation>
            <search><![CDATA[$language->get('text_pdf_sku'),]]></search>
            <add position="after"><![CDATA['act_size' 		=> $language->get('text_pdf_act_size'),]]></add>
        </operation>
  </file>
  
  <file path="catalog/language/en-gb/extension/module/hb_pdfinvoice.php">
	<operation>
            <search><![CDATA[$_['text_pdf_sku'] ]]></search>
            <add position="before"><![CDATA[$_['text_pdf_act_size']            	= 'Size';]]></add>
        </operation>
  </file>
	
</modification>