Depends on what you mean by "work with them offline". If you mean store the documents so you can access them if not connected to the internet, you already have it: DropBox.you can download files from the cloud an use them offline. You will need an app to handle the zip files: I use File Manager, but there are others. File Manager also intergrates with dropbox, Google Drive, and iCloud. If you only need to read them, these apps will take care of that for you.
As for editing, there are a few options. All have their limitations.
Some possible apps to look at:
- Pages, Numbers (Apple)
- Document (by Savy soda)
- document 2 go
- Smart Office
Make sure to read botht he descriptions and the reveiws carefully