How to backup?
All data and documents are copied to the respective files that end with ". receipts". In order to create a backup, this file must be backed up. Usually, the Time Machine does its job in the background when it is activated. However, it is also possible to duplicate the file or make a copy elsewhere, such as on an external drive or server. It is important that "Receipts" is closed while the copies are being made.
While any data is copied to the Receipts library I still recommend to keep original source files around. You can also open them by reference from within Receipts then.