pdf.txt 中为base64字符
例如：'JVBERi0xLjQKJeLjz9MKNCAwIG9iago8PC9Db2xvclNwYWNlL0RldmljZVJHQi9TdWJ0eXBlL0ltYWdlL0hlaWdodCA1MzEvRmlsdGVyL0RDVER................................................'
import base64
file = open('./pdf.txt', 'r')
x = file.read()
with open('temp.pdf', 'wb') as f:
f.write(base64.b64decode(x))
