Using the Files app you can import/export or copy from any file provider installed on your iPad, like Dropbox, Google Drive, Box, etc.
For example, you can install the Dropbox as a document provider on the Files app. Install the official Dropbox (or One Drive, etc.) app the open Files app then tap “Edit” button and add the Dropbox option. Open the Notes Writer then tap “+” button and choose the “Copy from Files” option. You will be able to browse the files including Dropbox location. You can also go to the “In Files” location on Notes Writer and edit in-place files directly from Files document picker.
You can also send to others apps, just select your document, tap the share in button the select “Send to App”.