a question about Phototransistor OPB608A
Pin 3 is the anode of an LED whose cathode connects to pin 3 and thence to ground. This means R3 supplies current to the LED and therefore WILL always show about 1.7 volts from pin 3 to pin 4. The forward volt drop of the LED is specified in the data sheet as 1.7 volts so no problems there.

This LED voltage will not change if paper or some other light-blocking material is brought in front of the device - it's a light emitting diode.

Pin 1 is the collector of the phototransistor and it's emitter (pin 2) connects to ground. The collector is connected to 5 volts via the 7.5k resistor R2. This part of the device is sensitive to light and pin 1 will flicker between 5V and 0V depending on the amount of incident light.

My advice is read the data sheet.