The sensors permission includes
an accelerometer, gyroscope, compass, barometer, thermometer and any other sensors present on a given device.
The permission is enabled by default for all apps because it can break compatibility.
I haven't found a need to have the sensors permission for the PDF reader. You can safely disable it if you like.
The only user app I enable the sensor permission for is the Camera app so that the app can get gyroscope data and tell whether I am taking a photo in portrait or landscape mode.
It's not recommended to disable the sensors permission for any system apps.