Hi,
The plugin is working fine and is being used by many.
Could you provide a screenshot of the settings of the plugin ?
Are you able to get the invoice from the backend order details page with the "invoice" button ? That's the first thing to try.
Regarding your screenshot, it looks like the browser has troubles displaying the print prompt. It's not clear whether it is related to the plugin or not. If you disable the PDF invoice plugin, do you still have the same issue ? If so, it would confirm it is not related to the plugin. If not, it means the plugin is providing the PDF but the PDF reader has problems reading it. It could either be a problem with the PDF reader or with the PDF file generation.