You can adjust invoice appearance by change fonts family and fonts sizes. This options affects also on Quotation, Proforma, Statement and Packing Slip.
For better appearance and readability you can show or hide horizontal and vertical lines in items table. You can use different combinations and choose appropriate for you.
Show empty rows option makes that items table has fixed size (10 rows). If on invoice is less than 10 items there will be added empty rows to 10 size. If there is more than 10 items table will be extended.