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<?php

add_filter( 'woocommerce_thankyou_order_received_text', 'wpb_thankyou', 10, 2 );

function wpb_thankyou( $thankyoutext, $order ) {
	$added_text = $thankyoutext . '<p>You can access the PDF Download from the <a href="/account-page">My Account Page</a>.</p>';
	return $added_text ;
}

<?php

add_filter( 'woocommerce_thankyou_order_received_text', 'wpb_thankyou' );

function wpb_thankyou() {
	$added_text = '<p>You can access the PDF Download from the <a href="/account-page">My Account Page</a>.</p>';
	return $added_text ;
}