I can help you here. If you go to the admin and select Settings > Billing, you can click on any of your bills listed here:
This is just an example from my test store to give you an idea! Once you click into your bill, you will see an option to download the bill or export it via CSV. If you click the 'download' button here:
This will let you download the bill in a PDF format:
Let me know what you think!
Social Care | help.shopify.com