There should be a [Done] button on the top left of the screen when viewing email attachments, if it is missing then you need to reactivate the menu bar.
To reactivate the menu bar, either tap into the attachment, or if that doesn't help, zoom out first and then tap into the attachment. Zooming out fixes most glitches, where you cannot go back to the previous slide.
If zoom out does not solve the problem, then reboot... to reboot your iPad hold down the on/off and home buttons until you see the White Apple Logo.
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