You can enable the attachment of PDF receipts in the email sent to the donor. To enable this feature, follow these steps:
Log in to your organization's Donorbox dashboard.
Click the Receipt Settings link underneath 'Account' in the left navigation bar.
Click the box that enables PDF receipts, then hit the update button.
Once that's done, a PDF copy of the receipt for all future donations will be sent to your donors along with the email receipt.