Interiors are often rejected for something mechanical: a number in the file that does not match the one Amazon expects. None of it shows in a PDF reader, so a failed upload is often the first sign.
01 An odd page count
This one is not refused. Every sheet has two sides, so KDP rounds an odd page count up to the next even number, and the page it adds is one you did not place. A cover made for the odd count is then one page short at the spine. Add one blank page at the back yourself, with nothing on it: a last page with a running head and a page number looks like a mistake.
How to check: if the page count is odd, add the page before you make the cover.
02 The page size is not the trim you selected
The PDF must match the trim size chosen at upload. The usual failure is a 6 × 9 book with a US Letter or A4 interior, written in Word at its default page size. Word's default is Letter in the US and Canada and A4 in most other countries.
How to check: look at the page size in your PDF reader's document properties. 8.5 × 11 or 8.27 × 11.69 (A4) for a 6 × 9 book is the problem.
03 Bleed declared and not drawn, or drawn and not declared
Bleed does not change the trim size. It enlarges the document by 0.125″ at the top, bottom and outside edges, never at the spine, which is not cut. A 6 × 9 book with bleed is a 6.125 × 9.25 document.
Ticking bleed at upload for a 6 × 9 file, or leaving it unticked for a 6.125 × 9.25 file, makes the two disagree.
How to check: the page is exactly 0.125″ wider and 0.25″ taller than the trim, or exactly the trim when you meant to use bleed.
04 The inside margin is below the minimum for your length
The minimum gutter grows with the page count, from 0.375″ for a short book to 0.875″ for the longest. A margin that was legal can stop being legal when the book gets longer. The two depend on each other, which has its own page.
How to check: look up the final page count in the table, not the count you had when you set the margins.
05 Fonts named but not embedded
A PDF can name a typeface without carrying it. It looks right on your computer, which has the font. The printer substitutes another, usually of a different width, so lines break in new places and the page count shifts.
How to check: in your PDF reader's document properties, every font should be listed as embedded.
06 More than one page size in the file
Usually a landscape page holding a wide table, or a page pasted from another document with its own size. Every interior page must be the same size.
How to check: look for a table or image that would not fit on a normal page.
07 The file is encrypted
A password, or restrictions on printing or copying, usually left by an export preset. KDP rejects the file before reading anything.
How to check: the PDF reader shows a padlock, or the document properties list restricted permissions.
08 The page count is outside the printable range
A paperback interior needs at least 24 pages. The most KDP prints depends on the paper and the trim. At the common sizes it is 828 pages on white paper and 776 on cream, and standard color takes 72 to 600. The largest trims take fewer: 590 on white at 8.5 × 11. A larger type size or a smaller trim brings a short book into range; another paper or a second volume, a long one.
How to check: compare the page count with the range KDP lists for your paper and trim. The PDF checker below does it for you.
09 Images too coarse for the size they print at
This does not cause a rejection, but the printed images look blurred. What matters is pixels per inch at the printed size. A 600-pixel image is enough at two inches wide and too little at four.
Print needs 300 dpi at the printed size. Phone and web photos enlarged to fill a column often reach half that.
How to check: divide the image's pixel width by its printed width in inches. Under 300, the image will look blurred.
Catching these before Amazon does
Seven of the nine can be read from the PDF: page count, page size, bleed, embedded fonts, consistent page size, encryption and page range. The print PDF checker reads all seven in your browser, without uploading the file. It also reads the final page count the gutter minimum depends on.
Image resolution is the one it cannot check, because it depends on the size each image prints at. The checker says so in its report.
A clean report is not a guarantee of acceptance. It means the file met the published requirements as of the date shown.