Pdf xml in Magento 2?

Question

When developing a payment fee, I want to add a surcharge fee to Pdf. I found the pdf.xml config. However, I don't know how it works. Can anyone help me to explain the config pdf xml.

vendor/magento/module-sales/etc/pdf.xml

<totals>

        <total name="discount">
            <title translate="true">Discount</title>
            <source_field>discount_amount</source_field>
            <title_source_field>discount_description</title_source_field>
            <font_size>7</font_size>
            <display_zero>false</display_zero>
            <sort_order>200</sort_order>
        </total>

UPDATE: Good explain here: Magento 2: How to Read From Configuration Tree

Answer 1

If you refer vendor\magento\module-sales\etc\pdf.xml

....
<totals>
    <total name="subtotal">
        <title translate="true">Subtotal</title>
        <source_field>subtotal</source_field>
        <font_size>7</font_size>
        <display_zero>true</display_zero>
        <sort_order>100</sort_order>
    </total>
    <total name="discount">
        <title translate="true">Discount</title>
        <source_field>discount_amount</source_field>
        <title_source_field>discount_description</title_source_field>
        <font_size>7</font_size>
        <display_zero>false</display_zero>
        <sort_order>200</sort_order>
    </total>
    <total name="shipping">
        <title translate="true">Shipping &amp; Handling</title>
        <source_field>shipping_amount</source_field>
        <font_size>7</font_size>
        <display_zero>false</display_zero>
        <sort_order>400</sort_order>
    </total>
    <total name="adjustment_positive">
        <title translate="true">Adjustment Refund</title>
        <source_field>adjustment_positive</source_field>
        <font_size>7</font_size>
        <display_zero>false</display_zero>
        <sort_order>500</sort_order>
    </total>
    <total name="adjustment_negative">
        <title translate="true">Adjustment Fee</title>
        <source_field>adjustment_negative</source_field>
        <font_size>7</font_size>
        <display_zero>false</display_zero>
        <sort_order>600</sort_order>
    </total>
    <total name="grand_total">
        <title translate="true">Grand Total</title>
        <source_field>grand_total</source_field>
        <font_size>8</font_size>
        <display_zero>true</display_zero>
        <sort_order>700</sort_order>
    </total>
</totals>

In this tags, <souce_field>, it gets from the fields of sales_order table in the database.

If you see your database sales_order table, you should find discount_amount field.

Each order has the values of totals' fields including discount_amount because your discount module saved the value into the field discount_amount.