If you want to see all the outstanding invoices on a project, you can accomplish that by using filters in the Invoices screen:

1. Filter the screen by project and select the project whose outstanding invoices you want to check. Check the Core Help for details on applying filters in the Invoices screen. 
2. Now, add the filter Open and select Yes for it.
3. Click on Apply Filter. Core displays all outstanding invoices for the project in the filtered view.