I did some experiments with sideloading PDF and mobi/prc books into my KF from my KDX.
I took a few PDF'S, and copied some into the "Books" folder on my KF; I took some other PDF's and copied them into the "Documents" folder. It didn’t matter what folder I put the PDF’S in (“books” or “documents”), they appeared under the “Docs” selection on the KF. They had "cover" images that consisted of the PDF filename displayed on a white background.
I then tried the same with some mobi and prc format books acquired from non-Amazon sources (these are easy to identify because they don't include an Amazon ASIN number in their filenames). No matter what folder I copied prc/mobi files into, they appeared under the "Docs" selection on the KF. Interestingly, the mobi files had "real" cover images, the prc books did not -- they just had the text of the book title on a white background.
The only books that appear under the "Books" selection are "Amazon" books. The books selection also has a filter at the top of the page which lets you display Amazon books resident on the cloud, or just those resident on the physical KF device. I presume the KF can distinguish which files in the folders go where based on the presence of the ASIN number in the filename, which if present, means the book will be displayed under "Books" -- all others will display under "Docs"
There is a welcome letter in the “documents” folder from Bezos. It is a .prc file; it appears on the carousel, but not in either “Books” or “Docs” on the KF. Also, the KF User Guide appears under “Docs” on the KF, but I cannot find it in any folder on the KF when I attach the KF to my laptop.
Under the “Books” folder is a folder called “Covers” which contains one file for each Amazon book actually resident on your KF. The name is the same as the first part of the Amazon book file name (which is the Amazon ASIN id?). You can open the cover file in MS Paint. It will be a 96 bpi image of the cover 160 bits high and 100-123 bits (as far as my samples showed) wide. There doesn’t appear to be any such cover facility for the documents folder (although the KF User Guide which appears under “Docs” does have a cover image — but then how the user guide resides on the KF is itself obscure).
So it appears that folder residence on the KF is only loosely coupleD with where it is displayed on the KF (if at all).