Within Teamwork Projects, in the Completed Tasks section of the Everything area, you can view all completed tasks across all your projects, and their details.
You can click on the task name to go to the task details screen, and also log time on the task via the clock icon to the far right of the task.
You can filter or sort the completed tasks as desired. Set the filters to your requirements and click update to refresh the list of completed tasks shown.
You can export a copy of this list of completed tasks to PDF or Excel from the Export button near the top right of the page.
For more information see: Everything section - All options