The interior file is where most rejections happen, usually because of page size or margins. A few settings fix almost every case.
Set the page size
Your page size must match the trim size you picked in KDP. For a 6 x 9 inch book, set the page to 6 x 9 inches. If any image or colour reaches the page edge, you need bleed: widen the page by 0.125 inches and make the height 0.25 inches larger. A book with no edge-to-edge images does not need bleed.
Apply the margins
The inside margin (gutter) depends on page count because thicker books curve more near the spine. KDP's minimums are 0.375 inches for 24 to 150 pages, 0.5 for 151 to 300, 0.625 for 301 to 500, 0.75 for 501 to 700 and 0.875 for 701 to 828. The outside, top and bottom margins must be at least 0.25 inches, or 0.375 inches when the pages bleed. These are minimums, and most books look better with more.
Embed fonts and flatten the file
Export with fonts embedded, or the printer may substitute them. Avoid transparency effects and form fields. Keep one page per PDF page, in reading order, with no printer marks or crop marks.
Handle images
Use images of at least 300 pixels per inch at the printed size. For black-ink books, convert images to greyscale, which also makes the file smaller. Check that no text overlaps the gutter.
Test before you upload
Run the file through the interior PDF checker. It reads the page count and page size in your browser, so the manuscript never leaves your computer. Then open the file in a PDF viewer and scroll through every page once. Look for stray blank pages, because each one adds to the page count and changes the cover size.
After you upload
KDP's previewer flags margin problems. Fix every warning, not only the errors, then order a proof. See our margins and bleed article for more examples.