You can use the filters on the left of the open orders screen to hide orders that have had their invoices printed.
- Expand the 'Flags' Group
- Inverse select (right click) the 'Invoice Printed' node
Another option would be to use Batch Pilots to print Invoices and enable the condition 'Invoice Not Printed'
Documentation for Batch Pilots can be found here ~ http://www.linnworks.com/Doc/batch_pilots