LINQ Query help

Hello :

I have a list

This Linq statement does not work. But that's the Idea

 DBDocuments.Where(x => DBDocumentAdditional .Find(a => a.DocumentId == x.DocumentId)).ToList().ForEach(t => t.Additional = DBDocumentAdditional );

Model:

public class Document
{
        public long DocumentId { get; set; }
        public List<DocumentAdditional> Additional { get; set; }
}

 public class DocumentAdditional
    {
        public long DocumentId { get; set; }
        public string Name { get; set; }
        public string Value { get; set; } 
    }

List<Document> DBDocuments = new List<Document>();

List<DocumentAdditional> DBDocumentAdditional = new List<DocumentAdditional>();

I populate DBDocuments  list with some data from database.This List has a key which is DocumentId.

I populate DBDocumentAdditional list with some data from database.This List has a key which is DocumentId.

Now I want to use LINQ to add the DBDocumentAdditional list  to the DBDocuments  list, for the Matching DocumentId's. List within a List with Matching Id

I am kind of stuck on syntax to do this. Please suggest, or help.

Thank you,