Dropbox Document won't Close

Running Dropbox v 2.1.1 on my original iPad (iOS 5.1.1)

Have a series of password protected PDFs in my dropbox - scans of passport, driver's license etc

When I open one, no problem until I try to close the document - unable to do so.

It remains open (& unprotected) on my iPad even if I quit DropBox (Hit the Home Button).

And on reopenong Dropbox, it is still open.

Unable to open any bars beneath the document to access a close function.

On my Iphone, the document closes when you tap on the Left-Arrow Dropbox icon at the top of the screen.

There is no such icon on the iPad version.

Appreciate any tips!

...John

PS When I open DropBox on my iPhone or Mac Pro, the document is not open - remains protected in the folder.