The reducing character of hydrides of group 15 elements increases from top to bottom because the strength of E-H (Here E= N, P, As, Sb, or Br) bond decreases down the group due to increase in the size of the central atom.
The stability of hydrides decreases down the group from NH3 to BiH3. This is due to a decrease in their bond dissociation enthalpy. Reducing character is directly related to the ease of losing hydrogen. NH3 is a mild reducing agent and BiH3 is having the strongest reducing character amongst all hydrides of group 15.
The trend of group 15 hydride on the basis of reducing character is as given below
BiH3 > SbH3 > AsH3 > PH3 > NH3
Stability decreases down the group as the bond dissociation energy decreases. Hence the bond dissociation energy decreases down the group due to the increase in the size of the central atom.