[BritTim]

When I download an invoice:

1. I can open it with Chrome and it looks basically OK.
2. I can open it with Microsoft Edge, and it looks basically OK.
3. I can open it with PDF Element, and it looks basically OK.
4. I get a similar error to what you are seeing when opening it with Adobe Acrobat. There is obviously something in the file that Adobe does not like.

You may want to raise a support ticket.