Thanks Graynomad! I am learning more in this forum 
The part of the pics is solved (that part is not a big problem now). I will send from Pics to Arduino a value type Float. For example, the smaller value that I can obtain with the measure of the Pic's ADC(10bits) is 5V/1024 =0,00488V. The smaller value that Arduino will receive by serial port is that. Will the arduino's serial port receive packet of the 8bits? I will send to Arduino the value in string form by pic's serial port. If later I need send by arduino's serial port (to other platform, for example my laptop) that smaller value I can use a simple "print()" ?
I wish that you understand that explanation. Thanks for your responses.