protected function insertTotals($page, $source)
    {
        $order = $source->getOrder();
        $totals = $this->_getTotalsList();
        $lineBlock = ['lines' => [], 'height' => 15];
        foreach ($totals as $total) {
            echo "<pre>";print_r($total->getData());// I get custom row values here
            $total->setOrder($order)->setSource($source);//But those get lost here

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Sales:etc/pdf_file.xsd">
    <totals>
        <total name="custfee">
            <title translate="true">Custom Fee</title>
            <font_size>7</font_size>
            <display_zero>true</display_zero>
            <sort_order>100</sort_order>
        </total>
    </totals>
</config>

$total->setOrder($order)->setSource($source);

$total->getOrder()->getData(); // Order data
$total->getSource()->getData(); // Invoice data
//Get specific data from total object
$total->getSourceField();
$total->getSortOrder();