You can view all of your previous purchases within the platform. You can also easily download the invoice for each purchase. See how by reading this article:
Step 1: Click the settings icon on the left hand side menu bar and then select the ‘Manage account’ tab
Step 2: Select the ‘Billing History‘ tab.
Step 3: This will show you a list of all your previous purchases, including subscriptions and documents. Simple click on the ‘Download’ button next to the invoice you are wanting to view/save.
This will then download a copy of the invoice to your device.