the method return a binary data encoded in base 64, what you need to do is decode the binary data.
see this article about enconde and decode binary data.
https://code.tutsplus.com/tutorials/base64-encoding-and-decoding-using-python--cms-25588
the Python client returns a xmlrpc.client.Binary object so you need to work with that object here an example using the Python client and Python 3
#!/usr/bin/env python
import SoftLayer
import xmlrpc.client
import base64
import os
USERNAME = 'set me'
API_KEY = 'set me'
quoteId = 1560845
client = SoftLayer.Client(username=USERNAME, api_key=API_KEY)
accountClient = client['SoftLayer_Billing_Order_Quote']
binaryData = accountClient.getPdf(id=quoteId)
decodeBinary = binaryData.data
file = open('test.pdf','wb')
file.write(decodeBinary)
Regards
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