Deeply nested search through Document tree
I find recursion complicated You can use a while loop:

private string SearchDocument(Document doc)
{
    while (doc != null) 
    {
        if (!string.IsNullOrEmpty(doc.Ordnungsbegriff))
        {
            return doc.Ordnungsbegriff;
        }
        doc = doc.ParentDocument;
    }
    return null;
}


Edit: As a further point - it's better not to mix languages in your code: Ordnungsbegriff should be Keyword (according to Google).