If you’re getting an error from KDP that your PDF is the wrong dimensions, the solution is easy! You can use Photoshop to crop each page to the correct size. I prefer the cropping method over resizing, since resizing can stretch your art and ruin the proportions.
To resize your PDF pages in Photoshop, first you need to find the correct size for your PDF. You can go to the Amazon KDP help page, Set Trim Size, Bleed, and Margins, and under Bleed, look at Examples of Page Size With and Without Bleed. Here you can look for the size that you want your book to be, and then on the right will be the exact dimensions you need to make each page.
Go into Photoshop and press C for the Crop Tool. Then, set the Crop Tool Properties to the dimensions that you found on the KDP help page (and set the pixels per inch to 300).
Now, you can open the PDF in Photoshop and select ALL of the pages, and click Open. You can now crop each page using the crop tool – and since you set the dimensions in the properties, each page will come out to be the perfect size! Be mindful of how much you’re trimming off – the text should be at least 1/4″ inside the page (you can eyeball it using the ruler along the top edge – press Cmd/Ctrl R if you don’t see the rulers).
If you have a spread in your book, make sure that for those pages, you’re not trimming any of the page near the spine. In other words, when you’re trimming the left page of a spread, you want your crop box positioned all the way to the right (so that only the left edge will get trimmed off), and when you’re trimming the right page of a spread, you want your crop box positioned all the way to the left (so that only the right edge will get trimmed off).
You can watch my video FIX KDP ERRORS: Resize Your Children’s Book PDF in Photoshop for a step-by-step tutorial on how to do this. :)