Help CenterLow and Medium Content books can NOT be uploaded as an eBook. People can not write on a eReader, and KDP will reject it. (If you have a written book please scroll down the page for help on that).
eBooks are generally for written books, (high content, Novel, Fiction, Non Fiction, Biography, etc) which are made for eReaders. eReaders have no ability to write in answers from a word search, crossword, and other puzzles type, or coloring books, or write into a planner or a journal. 98% of these eReaders are also Grayscale or Monochrome, how would users color on them?
For more information you can see this article Guide to Kindle Content Quality on KDP Help site. Scroll down to the section that says "Content unsuited for reading on Kindle".
It states this:
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Books that are not suited for Kindle may be rejected upon submission or removed from sale.
Some books are simply not well suited to the Kindle format. These include any kind of book where the main purpose is to allow the reader to write or color on the pages. These books are better suited to publishing in a physical form.
Examples of books not suited to the Kindle format include the following:
Puzzle books
Blank Journals
Pattern books
Coloring books
Facing page translations (left page in one language, right page is same content in another language)
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If you ignore this and still do it and a newer KDP employee allows your book to slip through and KDP gets complaints from customers, your account could be closed due to a bad customer experience. They are very against that happening to their customers.
WRITTEN BOOK HELP:
If you have a written book, that is a Novel, Ficiton, Non Fiction, Biograpphy, etc, and you have a PDF and you want to use the covert your pdf to do eBook. This is pretty easy with the help of a free tool that KDP offers Authors called Kindle Create. You really should do these types of books though as Reflowable Books, and not use the PDF way. But you can if you prefer simple and easy method.
If you made one of these types of written books in Book Bolt (we are not sure why you would since it is not a word processor and Microsoft Word or Google Docs would do it so much better), then save your project as RGB. Once done that, please see below.
To learn more about it please see these articles from the KDP Help.
Using this tool will allow the end result to be saved in .epub or .kpf format, which is the best format to upload as an ebook.
Create a Cover For eBook: You could do the cover in Book Bolt. Create a new project, pick custom, pick 6 x 9 (or whatever size your book is and one page. Then you create your front cover (no front and back cover in ebook).
Then click file in the Book Bolt Studio and pick export to PNG. You will need to convert that .png file to .jpg, just open it in a graphics program, even Microsoft Paint and click file and save as and pick jpg as the type. Or use good to search png to jpg converter. Upload that jpg as the cover.