Seems like Dropbox (https://www.dropbox.com/
) is an option, but with some work to get it to integrate with KDE: http://www.nixternal.com/kde-and-dropbox/
. Alternatively, and more secure, is SpiderOak (https://spideroak.com/
) which has a Qt client application. So that should integrate very well with KDE I think.
There are probably more options, but depends on what you need also. If you are using Chrome, Chromium or Firefox (but better Chrome/Chromium) as a browser, you can also try Mega (https://mega.co.nz/
), which at 50 GB free storage is hard to beat. But currently it only works through your web browser,