Each chapter of The Story of the World Activity Books includes a set of "Questions for Review" to guide a discussion between the parent and the student and assess student comprehension of the text. In the early grades, these should be done orally. Starting in about 4th grade, some parents want the students to write their answers. As a response to customer requests, we created the Review Questions Worksheets (formerly known as the Written Comprehension Responses) to save parents and teachers time and work. It is available only in PDF format. The PDF includes each Review Question and leaves space for the student to write a response.
The PDF is an optional supplement to the Activity Book. It does not replace the Activity Book; the answers (suggested responses) are included in the instructor portion of the Activity Book, but are not included with the Review Questions Worksheets.
The short answer: Review Questions Worksheets are intended for a subset of our customers. Adding the pages to the Activity Books would raise the cost of production and shipping for everyone. We made the PDF so interested customers could get the supplement at minimal cost.