Kindle Cloud Reader and Kindle for PC are excluded from Send-to-Kindle books. None of them will appear for download/reading. Although I think I have heard that downloading for another device and adding the file to Kindle for PC will allow it to sync, still, that is a ridiculously awkward workaround.
The reason appears to be a closely guarded mystery.
Only the smartphone/tablet apps and the Kindle devices (Fire/E-ink/Phone I guess) will download Send-to-Kindle books.
And this is a totally separate issue to the question of the PDOC vs. EBOK classification.
(Worth noting that Send-to-Kindle books are ALWAYS classified as PDOC, on any device. But usually they still show under Books.)