You can use the click activity and try to get the selector and make it dynamic by using a counter and loop thru all the check boxes on the page. So, after the checkbox is clicked, then another click for download. You may want to save the invoice using the customer name, which you can get using get attribute activity.
Once the loop is complete for a page, then you can use element exist activity to check if the next page link is active. If exists, you can use click activity to goto next page. If you have fixed number of pages, you can hard code that number to loop to that many number of pages.