Use the Figure.savefig() method, like so:
ax = s.hist() # s is an instance of Series
fig = ax.get_figure()
fig.savefig('/path/to/figure.pdf')
It doesn't have to end in pdf, there are many options. Check out the documentation.
Alternatively, you can use the pyplot interface and just call the savefig as a function to save the most recently created figure:
import matplotlib.pyplot as plt
s.hist()
plt.savefig('path/to/figure.pdf') # saves the current figure