Why did KDP reject my interior PDF?
Updated
The four usual reasons: the PDF's page size doesn't exactly match your chosen trim; fonts aren't embedded; the inside margin (gutter) is below KDP's minimum for your page count; or the page count is outside 24–828. Word exports commonly fail the first two.
Exact trim: if you chose 6×9 but the PDF pages are US Letter (8.5×11) — the default Word export — the previewer rejects or shrinks it. The PDF page size must be the trim size.
Fonts: 'font not embedded' means the file references a font without shipping it. Print PDFs must embed (usually subset) every font.
Gutter: KDP's inside-margin floor grows with page count (0.375" ≤150pp up to 0.875" at 828pp). Books near a bracket boundary fail when the margin was set before the final page count existed.
A typesetting tool that renders, counts pages, applies the right bracket, and verifies embedding removes all four failure modes — that's what a preflight report is for.