From Dropbox app, tap on the PDF, top right is an arrow going downwards into a 'box.' Tap this and choose Open in.
(I am looking at iOS6 and the latest version of Dropbox - they changed the layout recently.)
It will need to go into an app's document area - unlike a computer where it just sits on your desktop or in a folder for any app to open it, iOS requires it to be associated with an app (inside it.)
You can also connect your iPad to iTunes and drag in your PDFs into an app.
http://support.apple.com/kb/HT4094?v...S&locale=en_US
Does this help?
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