You can get a certificate of every individual work task you have completed OR a report of completed tasks from the desired timeframe. (note this is only available on the web version!)
How to get a report of multiple tasks:
Open "Payments & reports" from the homepage.
Go to "Reports" and select "Completed tasks report" as the report format.
Select the worker type and the desired timeframe for the report.
Finally, click "Download report" to download the report onto your computer.
How to get a certificate of an individual work task:
Open the history-page of your profile ("Orders" on the left side bar of the page -> and History-tab on that page.)
Select the task you would like to get the certificate for. Then, on the right edge of the page, there is a button saying "Get certificate".
Clicking that opens up a pop-up window where you can choose, whether you want to show the feedback from that task on the certificate or not.
Then just click "Download certificate"!