What makes a PDF 'print-ready' for KDP?
Updated
Four things, all checkable: every page sized exactly to your trim; all fonts embedded in the file; the inside (gutter) margin meeting KDP's minimum for your page count; and the page count inside KDP's 24–828 range. 'Looks fine on my screen' is none of these.
Print-ready is a property of the file, not the design. A beautiful interior exported at US Letter instead of 6×9 is rejected; a plain one with correct geometry sails through.
Fonts are the invisible failure: some export paths subset or skip embedding, and the press can't print type it doesn't have. Embedding is verifiable mechanically — no squinting required.
This is exactly what a preflight is for: each requirement measured against the file and shown pass/fail before Amazon ever sees it. Ceeryo runs KDP's geometry checks on every typeset book automatically.