Varry line thickness of pcb as resistor
It is not practical using conventional PCB technology 


The dimensions required to get values you would typically want would be impractical- the resistivity of copper is too low. This is not accidental, it's a good part of why we use copper as a conductor material.
Even if you could get a value you needed, the tolerance would be terrible, and trimming equipment is not cheap.
Copper is a poor choice for a resistor element- not only is the resistivity too low, it changes resistivity greatly with temperature (about 3900ppm/K) so it's more like a temperature sensor than a stable resistor. 
On top of that it tends to corrode in the presence of some typical pollutants so thin narrow structures would change in resistance.


It is possible in some cases to print thick film resistors on PCB substrate, but the PCB material limits the curing temperature so the resistors won't be very good (stable), and the tolerance won't be good either. Even the very cheapest SMT resistor is fired on a ceramic substrate at high temperature, trimmed to within a tolerance and coated to protect the element. 

Sometimes a very low value appears tempting to make with a PCB trace such as for a crude current limit shunt. Usually it's best to 'resist'.