Thanks a lot for your reply. I am hosting with SiteGround and they have SG Optimizer by SiteGround. SG Optimizer is a plugin that “allows to use the major performance optimisations for WordPress, which SiteGround hosting is providing on its servers”.
The SG Optimizer plugin has a PHP Config feature which runs a site check prior to PHP version change. It is this site check that lists WooCommerce PDF Invoices & Packing Slips as not compatible with PHP 7.1. and prevents the PHP switch.
Many thanks once again!