What is the maximum voltage drop on an LDO?
An LDO or any linear regulator can drop any amount of voltage as long as two conditions are met. First, the maximum input voltage is not exceeded. Second, the wattage dissipated by the regulator is taken into account. 

For example if using a 7805 with an input voltage of 25V with an output current of 100mA. The regulator is dropping 20V and dissipating (V*I) 2W. The wattage is quite high and would require extensive heat sinking to keep the regulator from burning up.