When WooCommerce PDF Invoices & Packing Slips runs out of memory, you’ll get an error that looks something like this:
Fatal error: Allowed memory size of ######## bytes exhausted (tried to allocate ### bytes)
This could happen on batch actions or very large orders, sites with a lot of plugins, little memory, resource heavy plugins, or a combination of all of these. PDF creation is a memory intensive job, especially if it includes several pages with images.
Go to WooCommerce > PDF Invoices > Status (tab), here you can find your WP Memory Limit (as well as some other things that could affect plugin performance). We recommend setting it to 128mb or more (See: Increasing the WordPress Memory Limit). Note that setting this value higher than what your hosting allows doesn’t have any effect.
If you have increased the WP Memory Limit, but still don’t see any effect, or if you’re already at the limit of your site host, you can try to optimize the plugin and/or find what other processes use your sites resources. Sometimes there’s just a single plugin, badly coded, taking a lot of resources.