Mit dem ftplib Package.
from ftplib import FTP
ftp = FTP('ftp.server.de')
ftp.login(user='user', passwd='pass')
remote_filename = 'aaa.pdf'
local_filename = 'bbb.pdf'
with open(local_filename, 'wb') as f:
ftp.retrbinary('RETR ' + remote_filename, f.write)