Pretty-printing in a viewer arranges text in lines, with indentations, to make the value easier to read. Pretty-printing is useful in Source view and AEPrint view. To give a simple example, pretty-printing makes the difference between this (no pretty-printing):

{1, 2, 3}

and this (pretty-printing):

{
    1, 
    2, 
    3
}

To control pretty-printing:

• Choose View > Pretty Print. If the menu item is checked, pretty-printing is turned on. Alternatively, use the Pretty Print checkbox within the viewer. (And the Apple Event Log window has a Pretty Print button in the toolbar.)