Glad to hear you still have them!
You can try downloading them from iCloud.com, putting a copy of each one in a "safe" place, and then try opening them with the Numbers/Pages apps. If they open ok, then you could try moving them into the iCloud Drive folders, replacing the existing versions there, and see if that works (again, making sure before you try all this that you've first put copies away in a safe place).
I've kept my documents in iCloud for several years now, without problems. For me a big advantage of doing that is the seamless sync with iOS, which you don't get with Dropbox.
Wherever you decide to keep them in the cloud, if they are important documents be sure to keep offline backups via Time Machine or whatever other system you like!
SG