What exactly are sensors , and why would an application such as GrapheneOS PDF reader need the permission to access them?

    LefoDpct

    The sensors permission includes

    an accelerometer, gyroscope, compass, barometer, thermometer and any other sensors present on a given device.

    https://grapheneos.org/features#sensors-permission-toggle

    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.

    Thank you, i somehow missed that piece of text on the product web-site.