I am about to install a new Franklin 4" pump (mod SM-3-22) in a 60 meter (200 ft) well.
The water outlet on the pump is 4 cm (1.5") diameter but the existing buried pipe from well to house is 2 cm (3/4") diameter. Would it be ok to fit a reduction adaptor on the pump for the smaller 2 cm pipe?
The pump specifications: L/H 2400 - 600 (10 GPM to 2.5 GPM); max lift - 105 meters (340 ft); RPM - 2850; CV - 1,00 (736 watts) (1 HP).
The overall height from well bottom to attic water tank is about 70 meters (230 ft).
I assume that the designation of 2400 - 600 L/H represents the range of performance of the pump. That would be somewhere in the range of a 7 GPM rated pump in the US, such as a Goulds 7GS07. The pumps in the US often have a substantial service factor on the motor so the 7GS07 might be pretty close to your pump.
You don't say if the tank is a pressure tank or a non-pressurized "gravity" tank. The mention of the attic suggests the latter. If a gravity tank, it would be best if the pump is controlled with a float switch rather than some kind of pressure swith/tank/float valve arrangement.
At 2400 L/Hr you will have about 25 meters of head loss per 100 meters of 2 cm pipe. At 600 L/Hr the loss will be less than 3 meters per 100 meters of pipe. Your total head loss will depend on the distance to the house.
If you must stay with the 2 cm pipe, you might consider getting a 1200 L/H pump if it will meet your needs and you have enough storage in the tank.
In any case, I would use 4 cm pipe in the well where it is not buried. If 4 cm is not available or too expensive, then anything larger than 2 cm would be better.
You should be aware that the restriction of the small pipe could result in a head/pressure of 105 meters (150 psi) at the point where the well pipe exits the water level in the well. Your pipe should be selected to withstand that pressure.
The system will probably work if you are careful to use pipe that will withstand the pressure. If I were selecting a pump for the system limited to the 2 cm pipe and a "gravity" tank, I would be selecting a smaller pump with less head, probably a nominal 1200 L/Hr rated pump with a 375 watt or 500 watt motor.