One of the questions that often comes up with Sharepoint regarding document libraries is "how do I link to a document stored in one library in others?" Unfortunately, there was no way to do this prior to the 2007 version without actually duplicating the document and uploading it into each library independent of each other. This obviously has ramifications by creating separate versions which will soon be out of sync. Changes made to one document won't be represented in the others.
To resolve this, SharePoint introduced a feature known as the Lint to a Document Content Type. If you aren't yet familiar with the concept of Content Types in SharePoint, you need to research this. I will attempt to post more on this subject later, but for now just understand that it is a way of defining the contents of a library beyond the document type that it was created as (.doc, .xls, etc...). The Link to a Document content type is a predefined type that creates a record in the library that will point you to another library. In order to use this feature you must first enable the use of content types within your library. To do this, you go to the settings tab of your document library and click the Advanced Settings link. Select Yes for Allow management of content types and click OK.
Now that you have enabled the ability to use content types, you must associate the content types you wish to use within this library. Back on the Document Library Settings page, you will now see a list of content types that are currently associated. By default Document will be enabled. Click the Add from existing site content types.
Choose the Link to a Document selection from the List Box and click Add >. Once you've done this you will now be able to go to your document library and click the new button and you will now see Link to a Document in the menu. Click this to provide a name and url for the document.
Although this may seem like a small thing, it gives us the ability to maintain a single copy of a document with all the features like versioning, workflows, and approvals that we chose SharePoint for in the first place while still allowing us to gain access to it in multiple places throughout the application.