Invoices
From WHMCS Documentation
The lists of invoices allow you to view and search all of the invoices in your WHMCS installation.
You can view these lists at Billing > Invoices.
Search Invoices
From the list of invoices, you can search using advanced search options, including searching by line item descriptions.
The global Invoices List lists invoices for all clients. Selecting a status is optional.
Invoices Tab
You can view the list of invoices for individual clients in the Summary tab in their client profile.