Welcome to Android Central! I responded in your separate thread that some apps have their own download folder. I wouldn't be surprised if Quickoffice or Kindle had their own download folder, although I haven't tried them myself. Use a file manager like ES File Manager to search for app-specific download folders.
EDIT: Ok, just tested out with Quickoffice. If you create a document on Quickoffice, it will ask you if you want to save it on Drive or in Downloads. If you picked Drive, then it would have saved in your Google Drive in the cloud, and wouldn't be in the Downloads folder. If you chose Downloads, then you should be able to access it by opening the Downloads app. Please note--if you're accessing the Downloads folder from within Quickoffice (by tapping the folder icon at the top), then you CANNOT delete the file there. You have to open the actual Downloads app in order to delete a file.