How do chopper driver work for stepper motor
The chopper has a sense resistor that is used to measure the current. When the current is below setpoint, then the transistor is ON. If the current rises above the setpoint then the transistors are OFF. This so called chopper, it is a two position controller ON/OFF.



As you can see from the picture, the voltage is pulsed, meanwhile the current is continuous and it is flipping around setpoint value.

Any assumptions regarding PWM in your question is wrong. Even when outputting a PWM signal, the current is always continuous, not pulsed.

The motor doesn't change the resistance, it rather builds up the back EMF voltage when it is spinning. So that's why you need 7.65V for 1.7A at standstill, and you might need 17.65V for 1.7A when the motor generates 10V.