Increase font size for readability

Me too! I found it in the FAQ file:

How do I quickly change the font of the invoice and delivery note?
You can change the font with CSS. Use the wcdn_head hook and then write your own CSS code. It’s best to place the code in the functions.php file of your theme.

An example that changes the font and makes the addresses very large. Paste the code in the functions.php file of your theme:

function example_serif_font_and_large_address() {
    ?>
        <style>
            #page {
                font-size: 1em;
                font-family: Georgia, serif;
            }

            .order-addresses address {
                font-size: 2.5em;
                line-height: 125%;
            }
        </style>
    <?php
}
add_action( 'wcdn_head', 'example_serif_font_and_large_address', 20 );