With the new version of Google Drive there is an amazing tool that will really help you.
Manage revisions... under the Menu
More when you select a file.
I won't explain to you here how it works because there is a very useful blog post that does it.
In this way the link won't change and viewers can just download or preview the latest version.
This is the link: Replace Google Drive file with new version
Try this alternative instead.
There's a nice option that can be useful: it is the
publishing that Google Drive offers.
It means that Drive will generate a link after publishing that can be automatically updated everytime you change your document. The published view is not editable by viewers.
You can stop publishing easily. The bad thing is that anyone who has the URL to your publication can view them no matter what visibility option you’ve chosen for the original one.
I've checked that you can only require viewers to sign in with their account if you're in the same company. I can't explain better but you can see the check box that appears when you publish it.
This is the support link for this option: Google publishing