I use G-Mail with my Ipad 2. When an E-Mail has an attachment, I go to it.
When I am done reading the attachment, I would like to return to the list of E-Mail messages, but I do not see a DONE that I can click on. How can I return to the list of E-Mail messages without going back to the Home Page?

Tap in the middle of the screen. The Done button should show up at the top left. It won't hang around long. If the document is zoomed this doesn't always work. Try use the pinch gesture to shrink the document back to normal size, and tap in the center again.