Your monthly email receipt from MemberMouse serves as an invoice. If you have particular requirements about what information needs to be included in that email, you can follow the instructions outlined here: Customize Billing Information on MemberMouse Receipts
Receipts in MemberMouse are in email format - we don't currently support receipts or invoices in PDF format. However, in Windows 10 as well as most versions of MacOS and Linux you can select the email text and print to PDF. For more information, view the Print to PDF article from Adobe.