Copy the .obj file that you want to view (and associated .mtl file, if any) onto
a directory named “Models” on the SD card of the Android device. It should then
appear in the list that is shown when you select “Open...” from the options menu
in ObjViewer.

You can interactively adjust the orientation of the object by dragging across
the view; drag across the middle part to rotate about the X (horizontal) and Y
(vertical) axes, and drag around the periphery to rotate about the Z axis
(perpendicular to the view) as well.