I use a Note 8, and when I try to upload documents from Dropbox with the browser, it tells me "file cannot be opened".
It seems that you've selected Dropbox as the default for uploading files. You can test this by going to
Settings -> Apps -> Dropbox and tapping
Clear defaults. Now, when you open your browser and try to upload files, it should ask what program you want to use to select the file.
In my personal opinion, you should only upload files from your device, since not all apps support uploading via the browser. If the file you need to upload is only on Dropbox, you can download it from the app by long-pressing the file and selecting
Export -> Save to SD Card.