By default any Expenses logged toward a Project that are marked ‘Billable‘ and have a Status of ‘Approved‘ are invoiced. To adjust this functionality see Company Settings in the Administration Guide
The Expense Billing Preference field on the Billing Configuration that is linked to the Project determines how the Expenses show up on any generated Invoice. Expenses can either be set to be included as line items on the same Invoice, or have a separate Invoice generated just for Expenses. In the first case Expenses will be displayed in the Invoice PDF that is generated, with the option to generate a separate PDF with only Expense line items.
