Final answer:
An invoice is the document that notifies the purchaser of their obligation to pay.
Step-by-step explanation:
The document that typically notifies the purchaser of his or her obligation to pay is an invoice.
When a purchase is made, the seller sends an invoice to the buyer, outlining the details of the transaction and the amount owed.
The invoice serves as a formal request for payment and acts as a legal document that states the purchaser's responsibility to pay for the goods or services received.