A bound document needs a clear distinction between the edge held in the binding and the edge a reader turns. Using a larger left margin on every page does not express that distinction across a two-page spread. Plan the spread before making detailed header adjustments.

Obtain the intended binding allowance from the person producing the document. Record the paper size and whether the source will be handed over as ordinary consecutive pages. Do not assume that a facing-page editing view also supplies a printer’s required booklet arrangement.

In a Pages word-processing document, open Document, Document and select Facing Pages under Document Margins. Inside is the binding edge; Outside is the opposite edge. Set the required values there. In the Section tab, with nothing selected on the page, select “Left and right pages are different” under Headers & Footers. Check this setting in each relevant section.

For a page-layout document, Facing Pages is under Header & Footer Margins. Its margin controls concern headers and footers, not body-page margins. Document Body identifies the branch: selected means word processing. Use the controls appropriate to the existing document rather than converting it merely to find a similarly named option.

Inspect a pair of pages together

Review a spread containing substantial text on both sides. Confirm that the larger allowance faces the binding on each page, and that the outside edges remain comfortably readable. Use the actual longest header text in the sample, not a short temporary label.

For an alternating title-and-chapter design, read the left header and right header as a pair. Then inspect the next spread and a spread across a chapter boundary. These checks reveal whether the intended design continues through the sections that matter.

Leave sufficient room for the content after final editing. A header that fits before a chapter title is renamed may wrap or collide afterwards. Check the finished wording in its real position.

Review the delivery file with the intended producer before printing the full run. Confirm page order, binding edge and any separate imposition requirement. Keep the editable source and the accepted output together so later revisions can reproduce the same spread geometry without relying on remembered margin values.

Reference: Apple: facing pages in Pages.