How to modify print invoice button as a report dropdown button which contain list of reports(Template)

 I believe you’re asking how to add a menu button which can hold other action buttons.

This code came from here

public MyGraph()
{
    MyMenu.AddMenuAction(Item1);
    MyMenu.AddMenuAction(Item2);
}        

public PXAction<SomeDAC> MyMenu;
[PXUIField(DisplayName = "My Menu")]
[PXButton(MenuAutoOpen = true)]
protected virtual IEnumerable inquiresMenu(PXAdapter adapter)
{
    return adapter.Get();
}

public PXAction<SomeDAC> Item1;
[PXUIField(DisplayName = "Item 1", MapEnableRights = PXCacheRights.Select, MapViewRights = PXCacheRights.Select)]
[PXButton]
public virtual IEnumerable item1(PXAdapter adapter)
{
    throw new PXException("Item 1 clicked!")
}

public PXAction<SomeDAC> Item2;
[PXUIField(DisplayName = "Item 2", MapEnableRights = PXCacheRights.Select, MapViewRights = PXCacheRights.Select)]
[PXButton]
public virtual IEnumerable item2(PXAdapter adapter)
{
    throw new PXException("Item 2 clicked!")
}