One of our customers sent us a packaging specification for shipping product to them. After reading it, I found that it does not apply to the product that we supply. (We only ship one item to them periodically). Should I keep it on file as a document of external origin? Any ideas would be appreciated.

If it will never apply, I would say don't keep it. Why waste time with a document you will never use.

Maybe file it as obsolete and keep for however long you keep obsolete documents.

Tom