You can combine multiple invoices of a same client.  After generating invoices for different cases/matters (or projects) of a client (either one by one manually or using the One-Click Batch Invoice Generation feature), visit the "Clients" tab and select a desired client on the left side panel.  Then on the right side, click on "Invoices" tab.

If there are two or more invoices in the invoice list, a button captioned as "Combine Selected Invoices" will appear.  Clicking on the button will cause a pop-up to appear.  In the pop-up, select desired (or all) invoices to be combined.

This process will create a PDF document that will include all selected invoices under a common header and grand total at the last page.  PDF will also include bookmarks for easy browsing.