Documents associations across various projects

In my setup I have a special project that is acting like a DMS and all formally released documents are stored in there.

On some occasions, a requirement shall be linked to one or more of those documents. How do I do this?

There seem to be no option to attach a document from another project. I know I can re-upload the document to another project but then it may create a situation in which we no longer having a source of truth. And linking it by URL is a bit ugly.

Is there a simple solution to this?