Hello fdlprod,
Thanks for reaching out to us.
I have reviewed that given URL is displaying the Maintenance mode.
Related to your query, it might be possible that uploaded PDF does not have the read permission.
Can you please check that after uploading the PDF those PDFs have read/write permission.
Or you need to troubleshoot in the below file path with our plugin.
File Path: plugins\woo-product-attachment\public\class-woocommerce-product-attachment-public.php
Function name: wcpoa_send_file
You just need to print variable $fullPath.
After print above variable you just need to check that variable path is correct or not.
If not then, you just need to change the code and make path correct and also make sure that PDF path is readable.
I hope this helps. Let us know if still you need any further assistance. We are happy to help.
Thanks,