The easiest way to add documents is to click the plus sign in the header. Then the below popup will appear.
Now you need to choose the category you want to add the files to. You can import as much files in one go as you like. If all the files are related to the same document, then choose All files in one item. If you want that the service creates separate documents from each file, then choose Each file as a separate item. After that, you are able to add the metadata of the document(s).
If you add more than one file in one go and you add all of them as a separate item, a Mass edit tool opens. There you are able to add the same metadata to all of the documents you are adding. You can add, for example, all of your Sales contracts in one go: if the contracts have similar metadata (like all of them are sales contracts), add those metadata with the mass edit, and then edit the files one at a time to add the rest of the metadata.