I give my answer about order email. Other emails and pdf should works with the same logic.

You have to look at \vendor\magento\module-sales\view\frontend\layout\sales_email_order_items.xml

where the block with class Magento\Sales\Block\Order\Email\Items has the template \vendor\magento\module-sales\view\frontend\templates\order\items.phtml

In this template the line <?php echo $block->getChildHtml('order_totals') ?> calls the block with totals (always defined in sales_email_order_items.xml)

...
<block class="Magento\Sales\Block\Order\Totals" name="order_totals" template="order/totals.phtml">
...

This template start with

<?php foreach ($block->getTotals() as $_code => $_total): ?>

where $block is a Magento\Sales\Block\Order\Totals instance.

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 addTotal() or addTotalBefore(), in this way your custom field wikk be added to $this->_totals array and it will be printed from the foreach.

I give my answer about order email. Other emails and pdf should works with the same logic.

You have to look at \vendor\magento\module-sales\view\frontend\layout\sales_email_order_items.xml

where the block with class Magento\Sales\Block\Order\Email\Items has the template \vendor\magento\module-sales\view\frontend\templates\order\items.phtml

In this template the line <?php echo $block->getChildHtml('order_totals') ?> calls the block with totals (always defined in sales_email_order_items.xml)

...
<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): ?>

where $block is a Magento\Sales\Block\Order\Totals instance.

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 addTotal() or addTotalBefore(), in this way your custom field will be added to $this->_totals array and it will be printed from the foreach loop.