Google Docs has introduced a three-level system of privacy for documents stored in its cloud. Sensibly, the default level is "private," which means only the account holder can get access to the document.
The next level is "anyone with a link." This is the novelty, allowing you to share your Google document by sending a link; the system it replaced required you to email formal invitations to particular recipients. Now, as the label indicates, anyone who knows the link can see your document. Google analogizes this to an unlisted phone number, which relies on secrecy for security. Here is where it's important to pay particular attention. Once the link is shared with anyone, your document is no longer as secure as it was under the "by invitation only" system; any confidentiality it may have will depend upon that link's staying confidential in the hands of those you've sent it to.
The third level is "public on the web," which means that the document will be indexed by search engines and will show up in search results. This has some utility as a low-level means of publication short of an actual website.
As Google says, "Sharing just got easier." As we might say, "Privacy now takes more care."
I think you can reset the link at any time if the url escapes beyond who you intended it to go to.