Within Teamwork, you can view all completed tasks across all your projects in the site-level Everything area.
You can click the task name to go to the task details view. You can also log time on the task via the clock icon to the right of each task.
You can sort the completed tasks in a number of ways via the sort dropdown.
Click the filters button in the top right to open the advanced filters pane.
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