AttributeError: 'bytes' object has no attribute 'encode'; base64 encode a pdf file

I am trying to base64 encode a pdf in python. Several SO answers to this worked for other people but not on my end for some reason. My most recent attempt is:

# http://stackoverflow.com/questions/12020885/python-converting-file-to-base64-encoding
with open('/home/cchilders/projects/myproject/data/books/software-and-mind.pdf', 'rb') as f:
    encoded = f.read().encode("base64")
    print(encoded)

I get

AttributeError: 'bytes' object has no attribute 'encode'

How can I base64 this pdf file? Thank you