functions.php

<?php
function wc_pdf_watermark_extend_template_tags( $parsed_text, $unparsed_text, $order, $product ) {
	// Look for {product_title} in text and replace it with the product title
	$parsed_text = str_replace( '{product_title}', $product->get_title(), $parsed_text );
	return $parsed_text;
}
add_filter( 'woocommerce_pdf_watermark_parse_template_tags', 'wc_pdf_watermark_extend_template_tags', 10, 4 );