Can a voltmeter itself disturb the exact measuring of voltage?
Yes, an analogue voltmeter, by itself, could disturb the precise measurement of a voltage by loading the source and changing its voltage.
The characteristic of an analogue voltmeter, which would decide how much it, by itself, could load the source and change the voltage being measured, is known as its 'sensitivity'.
The sensitivity of an analogue voltmeter is expressed in ohms/volt. The best analogue DC voltmeter would have a sensitivity of 20,000 ohms/volt. Such a voltmeter, with a full scale reading of say 1.0 V, would present a load resistance of 20,000 ohms to the source. Should the actual voltage be 0.2 V, the current drawn by the meter from the source would be 0.2/20000 or 10 μA only.