Which of the following is NOT a type of formal testing?

Exploratory testing is characterized by its informal and unscripted nature, where testers actively explore the application to identify defects based on their intuition and experience rather than following a formal set of predefined test cases. This approach allows for flexibility and adaptability, making it particularly useful in situations where requirements may not be fully understood or may change.

In contrast, the other options mentioned—peer review testing, user acceptance testing, and document review testing—are all structured processes that follow specific protocols and criteria to ensure comprehensive evaluation. Peer review testing involves systematically reviewing a colleague's work to identify issues collaboratively, user acceptance testing (UAT) is conducted by end-users to validate that the software meets their requirements, and document review testing includes a detailed evaluation of project documentation to ensure it meets specified standards. These structured methodologies are aimed at confirming quality and compliance, contrasting with the exploratory approach specified in the question.