While creating invoices, you might want the project managers to review or update them before finalizing the invoices. This can be somewhat difficult, especially if some or all of your project managers do not have access to the billing modules in Core.
One way to accomplish this is to email the draft invoices (PDFs) to the project managers, which can be reviewed and and feedback sent to the billing person.
A second way to handle this is to prepare a pre-billing report that lists out projects by project managers. This report displays the contract amounts of the projects, which can be used by project managers to compare the approved time and expenses against. They can comment on this report before sending it back for invoice processing.
The third and easiest solution is to purchase Billing subscriptions for such project managers. Those project managers can access the billing modules directly and review the draft invoices before the billing person finalizes them.