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Hi @reflex84,
This 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?
To solve this, please go to WooCommerce > PDF Invoices > Status tab, and click on the Reinstall fonts button. Then, try to open an invoice and check if it looks good.
Let me know whether it worked, or you need more help!