[AnemicOak]

If you long press on a book in the Dropbox app and select export a menu should pop up. One of the options should be Save to SD Card so you can save them in the Kindle folder on the device (I'm assuming they're in Mobi/AZW or AZW3 format).

There is also an option in the export menu called Send to Kindle. This will open a dialog where you can pick a device/app registered to your account and send it via Whispersync and optionally save it to your Kindle cloud as well (this only works with Mobi's, not KF8/AZW3's).