Pdf Totals do not have a nice User Interface to change how they are sorted. The sort_order is determined by the etc/pdf.xml
files. For example in /vendor/magento/module-sales/etc/pdf.xml you'll find
<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>