Google-drive – Encryption for Google Documents

From this Google help entry, it would seem the urls are permanent until you reset them

Try to use cloudHQ service. cloudHQ service provides the following features:

• copy and move files and folders between Google Docs, Basecamp, and SugarSync (or Dropbox) directly from Google Docs interface

• browse and view Basecamp project attachments and SugarSync (or Dropbox) files directly from a Google Docs interface

• automatic backup of multiple Google Docs accounts and Basecamp projects to SugarSync storage (or Dropbox storage)

• continuous two-way synchronization of multiple Google Docs accounts, Basecamp projects, and SugarSync storage (or Dropbox storage)