By using "select all", the returned object is a classical array where dropShadowSettings isn't a valid property hence the error.
Instead of setting the props straightforwardly, I would recommend applying an object style. That way, you will be able to edit the style manually and see the previous concerned objects being updated.
var doc = app.activeDocument;
var os = doc.objectStyles.itemByName ( "myDropShadow" );
!os.isValid && os = doc.objectStyles.add ( {
name:"myDropShadow",
transparencySettings:{
dropShadowSettings:{
mode:ShadowMode.drop,
angle : .0083,
xOffset : 0.08,
yOffset : 0.08,
size : 0.6,
}
}
});
app.activeDocument.pageItems.everyItem().appliedObjectStyle = os;
By the way it's better not to use UI commands such as copy/paste/select as they are time consuming and there is always an alternative within the dom itself.