The first thing to check is which WooCommerce email notification you’re attaching the invoice to. You can verify this under WooCommerce > PDF Invoices > Documents > Invoice > Attach to:
Remember to check that you have the correct document type selected while in the Documents tab:
Some of these services either don’t allow attachments, have capped the attachment count to 1 document or have capped the file size limit to 1 MB.
In addition to answering the above question, please follow these steps to find out if the issue occurs before the message is sent: