I give my answer about order email. Other emails and pdf should works with the same logic.
You have to look at
where the block with class
Magento\Sales\Block\Order\Email\Items has the template
In this template the line
<?php echo $block->getChildHtml('order_totals') ?> calls the block with totals (always defined in
<block class="Magento\Sales\Block\Order\Totals" name="order_totals" template="order/totals.phtml">
This template starts with
<?php foreach ($block->getTotals() as $_code => $_total): ?>
$block is a
Then, to add your custom field to totals, try to write a plugin related to one public method of this class, i think you could choose
addTotalBefore(), in this way your custom field will be added to
$this->_totals array and it will be printed from the foreach loop.