Help! Our door closer slams the door

…My freeholder has just fitted a door closer at my request as the main door slams when people close it. However, the one they have fitted has not remedied the problem as the door needs to be closed very gently to eliminate the slam (and rattle of glass in the door).  Do you have a door closer which can move extremely slowly but also push a door closed??
If it is a hydraulic door closer they’ve fitted, it should have some adjustability on the closing speed.  Basically you want to slow down the speed and reduce, or turn off, the latching action.  Most door closers should offer this adjustability.  Our Geze TS4000 video shows the adjustment of this high use, heavy duty, fully adjustable door closer, which might give your fitter some assistance.  Any of the Geze door closers from TS1500 up (depending upon the width of your door) should give you a good result.