in the toolbar, then click the Section tab.A document section is a portion of a document, such as a chapter, to which you can apply different formatting from the rest of the document. For example, a section can have different page numbering, background images, headers and footers, and table of contents.
In thumbnail view, a yellow box appears around the pages in a section.
The control for creating a new section is at the bottom of the sidebar on the right. If you don’t see a sidebar, or the sidebar doesn’t have a Section tab, click in the toolbar, then click the Section tab.