If you go to the Task section and filter the tasks to 'Show completed' you will then see all completed tasks which are shown as 100% complete.
To then search for any key words in the completed tasks we recommend that you use the search function on your PC, by pressing both the Ctrl key and the F key down this will activate the search function.
Just type the keyword into the search bar which will be shown in the top right-hand corner of your screen and use the up and down arrows to scroll through the page to view any matches that are found.