Visit your user profile and click on the "Invoices" tab. If you use the Helpling app, please click on "More" in the navigation bar at the bottom and then on "Invoices". 
You have the option to download your invoices in PDF format.
In addition, you will also find the option to download a payment summary for an entire year. To do this, click on "Download annual invoice overview", then select the desired year and confirm with "Download summary".