The Kobo Forma is the only Kobo ebook reader with official support for Dropbox, but it turns out there is unofficial support for Dropbox and additional cloud services for other Kobo ereaders too, not just the Forma.
I came across a post at MobileRead where someone put together a set of scripts that allows Kobo ereaders to synchronize with a number of cloud services.
Currently it works with Dropbox, Google Drive, ownCloud, NextCloud, and pCloud.
There’s a project over at github called KoboCloud where you can find the latest downloads and installation instructions.
It looks like it takes a little work to get everything configured properly for the particular cloud service that you are using, but all the directions are there on the github page.
I have not tried this myself but there are a number of reports of it working in the thread at MobileRead. Some folks have some trouble getting things to work so it may be more of an advanced hack that requires some troubleshooting skills to get everything up and running than some of the other Kobo mods.
Nevertheless, it’s nice having an option to wirelessly add ebooks to a Kobo ereader, even if it’s not officially supported by Kobo.