Thank you for reaching out to Jotform Support. Can you provide with us the Specific URL of the form that you're having an issue with so that we could do some testing and further check it for you.
I have cloned your form to test and I have created a new PDF document.
Then, I have attached both invoice and my newly created PDF document and I was able to do so.
I have cleared your form caches. Could you please clear your browser's cache as well? Then, try to attach your invoice and submission PDF once again. Let us know how it goes.