Adding link to a document's properties (or linking 2 documents together)

Adding links to a document's properties is not bult into Sharepoint but there are several workarounds.

• If all documents are in the same library, you could add a multi-choice lookup field referencing the library itself and let the users select the documents they want to link to. Unfortunately, the UI for multi-choice lookup columns doesn;t work well for a large number of items. You can't rename the links either.
• You can create your own custom lookup field with the UI you want, if your project's time and budget constraints allow it.
• You can add an HTML text field and have the users copy and paste the links to the documents they want in there. It is ugly, it creates the possibility of broken links, but it is another option to consider if the other options are not suitable.