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You need a buffer because an Arduino can only supply 40mA per output pin, whereas the 5V version of the HV multiplier requires up to 250mA.
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However, there are very few op amps that can supply as much as 250mA output current. That is why I suggested using an op amp and NPN transistor as the buffer.
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You cannot connect the input of the buffer direct to an Arduino output pin, because the Arduino does not have an internal DAC to drive any output pins with a variable voltage, and you must not try to drive the HV multiplier with PWM. That is why I included the RC network at the input to the buffer.