Zooming enables you to increase or decrease the magnification of a view, while panning shifts the view within the view window.
Zoom In and Out
To zoom in and out, use one of the following:
Mouse scroll wheel (or middle mouse button if you have an older mouse)
Ctrl + left mouse button
Note: To change the mouse buttons used for zooming and panning, see Editing Mouse Button Assignments.
Zoom to Fit
To zoom the view so that the entire image is visible and centered in the view window,
click 
on the viewer toolbar.
Zoom Box
When your Left mouse button is set to ZoomBox in your Options, you can zoom in on a specific part or area of your model by simply clicking and dragging a box around it.
Note: By default, the Left mouse button is set to RotateXY. To set a mouse button to ZoomBox, see Editing Mouse Button Assignments.
Pan
To pan the view, use one of the following:
Right mouse button
Shift + left mouse button