It sounds odd that a device would output a PWM signal with a varying duty cycle and at the same time have a variable peak voltage.
Such a signal would be modulated in two ways with two independent signals. It is possible but odd.
So to measure the peak voltage you need a potential divider arranged such that the maximum voltage gives you 5V or less on the arduino. This will affect the resolution, if this is too much then you will need a higher resolution external A/D converter.
To get at the duty cycle information you could feed the signal into the base of a transistor ( through a resistor ) and have the emitter to ground and the collector to the arduino input pin, then enable the internal pull up resistor. Use the Pulse in function to measure the high and low periods of the signal.