Even experienced authors get tripped up because the upload form feels like one process, but the inputs are different. For ebooks, KDP primarily uses the EPUB file; for paperbacks, KDP uses your print interior PDF and a wraparound cover file that includes the front cover, spine, and back cover.
For the ebook file, validate that chapter breaks and headings appear where you expect, and that links (if present) don’t land on missing destinations. Also check how special characters, italics, em dashes, and quotation marks render—things that look fine in a word processor can shift in EPUB conversion. For the paperback, confirm the trim size matches your cover and interior settings, and that margins and page numbers land correctly. KDP’s preview tools catch many layout problems, but they can’t replace a physical proof when you care about how text density and page layout feel. If you changed fonts, sizes, or spacing late in the workflow, rebuild or regenerate the export files before you upload; otherwise the previewer may be showing an older version than the one you reviewed in your editor.