The PreTeXt Guide

An electronic PDF will default to page layout appropriate for a document printed single-sided, which makes the most sense for a document that may not ever be printed, or which possibly might be printed on a personal printer.

A print PDF will default to page layout appropriate for a document printed two-sided, which makes the most sense for a document that may be sent to a print-on-demand service or printed on a printer that will print on both sides of a sheet of paper.

These defaults may be overridden with the publisher entry latex/@sides switch with values one and two. See Subsection 44.3.2 for the specifics of this entry.

One-sided layout will default to symmetric left/right margins, and page headers with the page numbers always placed in the upper-right corner as part of default page headers. There will be no blank pages between chapters of a book.

Two-sided layout will have asymmetric margins with the ratio of inner (adjacent to spine) to outer at 2:3. You need some extra space at the spine to compensate for the binding, but when a book is open, the two pages are separated by two inner margins, so these do not need to be as wide as the outer margin to give some distance between the pages. And readers will want more space to write in the outer margins, perhaps providing simple proofs of important results. (This ratio may be changed with the hmarginratio key of the geometry package.) Page headers will have page numbers on the outside of the page, with odd numbers on the right-side page. Chapters will possibly have a blank page between them, so they begin on a right/odd page. Behavior is similar in the front matter and back matter.