Every document section has a set page settings used for all pages of the section. Page settings include the paper format (paper size), page orientation, page margins, and a few other characteristics.
When a document contains multiple sections, you can specify different page settings for each section. For example, you could specify larger page margins for the first section, or the landscape page orientation for the last section of the document. Note that toggling the "Mirror margins" or "Different odd and even headers/footers" options always affects the whole document, ie all sections of the document. Also the pages belonging to a contiguous sequence of sections separated with continuous section breaks, display with the page settings of the first section from that sequence.
To change page settings of the whole document or its portion, use the File | Page Settings... menu command. You have a choice to apply new page settings either to the whole document, the current section, the selected sections, or the selected text.
When you change page margins through the rulers, new margins get applied either to the selected sections, or to the current section.
Another way to change page settings of any section is the Sections panel of the Control Board.
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