Pages are not all the same size

One size for every page

Every page of a print interior must be the same size: the trim size, or the trim size plus bleed on a bleed interior. A single page that differs is enough for the file to be rejected, because the printer cannot impose pages of different sizes onto the same sheet.

How mixed sizes get into a file

Almost never on purpose. The usual routes are:

• Merging PDFs made in different tools — a title page from a design app, the body from a word processor — where one tool used US Letter and the other the book's trim.
• A landscape page for a wide table or chart, which is the right size rotated and still counts as a different size.
• Bleed applied to some pages and not others, so some pages are larger by the bleed allowance.
• Scanned pages or images inserted as whole pages at their own size.

Fixing it

Go back to the source documents and set the same page size in each before exporting, then merge again. Resizing pages in a PDF editor after the fact usually scales the content too, which shifts your margins and can push text into the gutter.

For a wide table, rotate the content within a portrait page rather than rotating the page. If any page in the book needs bleed, give every page bleed and keep text on the others within the bleed-interior margins.

The pre-flight checker reports the first page that differs from page one, so you know exactly where to look.

Page size versus content size

A page can be the right size and still look wrong: a PDF exported at the trim size with its content scaled down to fit leaves oversized margins, and one with content scaled up pushes text into the gutter. When fixing a mixed-size file, check the content position as well as the page dimensions, and compare a few pages against the margin minimums for your page count.