Change this:
@Html.ActionLink("Delete", "DeleteDocument", "Document",
new { id = document.Id, caseId = document.CaseId },
new { onclick = "return confirm('Are you sure you want to delete this document'"+
document.FileName +"'?');", @class = "button small button alert"})
to this:
@Html.ActionLink("Delete", "DeleteDocument", "Document",
new { id = Model[0].ToString(), caseId = Model[0].ToString() },
new { onclick = "return confirm('Are you sure you want to delete this document " +
document.FileName + "?');", @class = "button small button alert" })