Option Explicit
Sub Document_Open()
Dim strWorkbook As String, strColumns As String
Dim lngRowIndex As Long, lngColIndex As Long
Dim arrData As Variant
Dim oCC As ContentControl
  Application.ScreenUpdating = False
  strWorkbook = "D:\Data Stores\Populate Array from Data.xlsx"
  If Dir(strWorkbook) = "" Then
    MsgBox "Cannot find the designated workbook: " & strWorkbook, vbExclamation
    Exit Sub
  End If
  Set oCC = ActiveDocument.SelectContentControlsByTitle("ID").Item(1)
  arrData = fcnExcelDataToArray(strWorkbook, , , False)
  oCC.DropdownListEntries.Clear
  For lngRowIndex = 0 To UBound(arrData, 2)
    'Examples:
    '1. Populate the dropdown list text and value property using data from column 1
    oCC.DropdownListEntries.Add arrData(0, lngRowIndex), arrData(0, lngRowIndex)
    '2. Populate the dropdown list text property using data from column 1 _
        and the value property using data from column 2
    'oCC.DropdownListEntries.Add arrData(0, lngRowIndex), arrData(1, lngRowIndex)
    '3. Populate the dropdown list text property using data from column 1 _
    '   and the value property using concanated data from all remaining columns.
    'strColumns = vbNullString
    'For lngColIndex = 1 To UBound(arrData, 1)
    '  strColumns = strColumns & "|" & arrData(lngColIndex, lngRowIndex)
    'Next lngColIndex
    'strColumns = Right(strColumns, Len(strColumns) - 1)
    'oCC.DropdownListEntries.Add arrData(0, lngRowIndex), strColumns
  Next
lbl_Exit:
  Application.ScreenUpdating = True
  Exit Sub
End Sub
Private Function fcnExcelDataToArray(strWorkbook As String, _
                                     Optional strRange As String = "Sheet1", _
                                     Optional bIsSheet As Boolean = True, _
                                     Optional bHeaderRow As Boolean = True) As Variant
Dim oRS As Object, oConn As Object
Dim lngRows As Long
Dim strHeaderYES_NO As String
  strHeaderYES_NO = "YES"
  If Not bHeaderRow Then strHeaderYES_NO = "NO"
  If bIsSheet Then strRange = strRange & "$]" Else strRange = strRange & "]"
  Set oConn = CreateObject("ADODB.Connection")
  oConn.Open ConnectionString:="Provider=Microsoft.ACE.OLEDB.12.0;" & _
        "Data Source=" & strWorkbook & ";" & _
        "Extended Properties=""Excel 12.0 Xml;HDR=" & strHeaderYES_NO & """;"
  Set oRS = CreateObject("ADODB.Recordset")
  oRS.Open "SELECT * FROM [" & strRange, oConn, 2, 1
  With oRS
    .MoveLast
    lngRows = .RecordCount
    .MoveFirst
  End With
  fcnExcelDataToArray = oRS.GetRows(lngRows)
lbl_Exit:
  If oRS.State = 1 Then oRS.Close
  Set oRS = Nothing
  If oConn.State = 1 Then oConn.Close
  Set oConn = Nothing
  Exit Function
End Function