Low Pass Filter for 20 kHz PWM signal
Why figure 2 shows 0.7 of 1023 and cutting after ADC value of 700. Is
  it for opamp?


You almost certainly need to use a rail to rail (R2R) op-amp that is capable of producing a decent output close to the top power rail. If you are using something like an LM324, the soft clamping you see at the top of the waveform is likely due to it not being R2R on the high parts of the input signal and also on the high parts of the output signal.