If you have the dropbox app installed or something like readledocs, good reader etc. These allow you to access you dropbox docs, then rather than using open, which opens them in the apps native reader, you use 'open in' option and it will give you a list of compatible apps you can open that file in. E.g pages etc.
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That is really useful thanks. One question, how do I then save my edits in Pages back to dropbox? I have tried but so far I have failed.