import matplotlib.pyplot as plt
import numpy as np

data = np.random.normal(size=1000)

### correct pdf

fig = plt.figure()
ax = fig.add_subplot(111)

ax.hist(data, fill=False, log=False)

fig.savefig('plot1.pdf', format='pdf')
fig.savefig('plot1.png', format='png')

### wrong pdf

fig = plt.figure()
ax = fig.add_subplot(111)

ax.hist(data, fill=False, log=True)

fig.savefig('plot2.pdf', format='pdf')
fig.savefig('plot2.png', format='png')