Kindle Library: books/documents that I uploaded myself can't be downloaded to Kindle for PC

Kindle Library: books/documents that I uploaded myself can't be downloaded
to Kindle for PC

my Kindle library contains a number of books I bought from Amazon, plus
several documents I uploaded to Amazon so that I can read them on my
Kindle devices. This works on my Android phone and tablet, but the library
won't let me download these documents to Kindle for PC. Why not?