I use my work iPad for exactly this
I have a 100gb Dropbox account which my company pays for
You can open and view the documents in Dropbox but not edit, if you want to edit you can “open in” I open in Numbers and it downloads it to my iPad so I can edit or view.
Once it’s on your iPad you can edit/view without internet but of course you will need WiFi or 3G to download the document from Dropbox
If you want a portable option you could also look at the new Airstash from Hitachi-Maxell which gives you the option of storing document on a separate device (or SD cards) and transferring wireless
AIRSTASH