Add a gutter margin for bound documents
A gutter margin adds extra space to the side margin, top margin, or inside margins of a document that you plan to bind. A gutter margin helps ensure that text isn't obscured by the binding.
Go to Layout > Margins.
Select Custom Margins.
In the Gutter box, enter a width for the gutter margin.
In the Gutter position box, click Left or Top.
Note
The Gutter position box is not available and determined automatically when you use the Mirror margins, 2 pages per sheet, or Book fold option.