In SharePoint 2003 many users wanted to be able to have a document library the was used as a central repository even if the document was not housed in that library. In SharePoint 2007 this became a possibility with the improvement of content types. Document libraries now can have multiple content types, which allows us to add content types.
This post will focus on the specific steps to adding a Content Type, Link to a Document.
From your document library select settings and then Document Library Settings.
Look for General Settings. From there click on Advance settings. Here is where you will be able to set up the document library to have multiple Content Types.
In Advance settings you will need to change Content Types from No to Yes. Click OK.
Now it is time to add the Content Type Link to a Document (you can also add any other Content Type). Click on Add from existing site content types.
Here is where you would select and add the Link to a Document Content Type. Click OK.
Now you will see that you have a new Content Type of click to a Document.
You now have a selection to add a Link to a Document. this will allow you to have an actual document and a link to another document or site.