We use Stripe for billing, and have turned on the ability for customers to download invoices from the "billing portal." To get there:
1. Open your web app ( app.grouptrackcrm.com)
2. Click on your Name in the top right corner and select Billing and Upgrade
3. Click "Manage Plan"
4. Scroll down in the self-serve billing portal to see your invoice history.
5. Click the link next to the date of the invoice you would like to open (which will also give you the option to download the PDF)