While it still may take a very large time to bruteforce (close to infinity), it is not really smart to keep confidential documents protected that way. If you don't care who reads it then it doesn't matter. But I wouldn't put the specifications of your latest project on Google Docs.
You are also risking the fact that the links may leak, when authentication is needed you you can still prevent people from accessing your file. If, however, authentication is not forced, anyone that can get its hands on the link, can view the document.