This could be related with the plugin fonts, that often happens when you migrate your site, or when you or a cleanup process accidentally delete the plugin temp folder, located under wp-content/uploads, maybe one of these things happened recently?
If this is the case, please go to WooCommerce > PDF Invoices > Advanced > Tools tab, and click on the Reinstall plugin fonts button. Then, try to open an existing invoice or packing slip and check if it looks good.
Please note that we also run a daily check to reinstall the fonts if needed, but clicking the Reinstall plugin fonts button forces this routine and fixes it faster 🙂
Let me know whether it worked, or you need more help!
Could you please also send me a screenshot of your System Configuration and Write Permissions, that you’ll find under WooCommerce > PDF Invoices > Advanced > Status tab?
The above are reference screenshots of the System Configuration and Write Permissions areas.