Function MacroName(strID As String) As String
    'Run your workbook code here
    MacroName = "C:\path\" & strID & ".pdf" ' return the PDF path and name
End Function

Option Explicit

Public Sub xlGetData(olItem As MailItem)
Const strWorkBook As String = "C:\Path\Workbookname.xlsm" 'The name of the workbook
Dim xlApp As Object
Dim xlWB As Object
Dim strID As String
Dim strPDF As String
Dim bStarted As Boolean
Dim olReply As MailItem
Dim fso As Object
    strID = olItem.Subject
    On Error Resume Next
    Set xlApp = GetObject(, "Excel.Application")
    If Err <> 0 Then
        Set xlApp = CreateObject("Excel.Application")
        bStarted = True
    End If
    On Error GoTo 0
    Set xlWB = xlApp.workbooks.Open(strWorkBook)
    strPDF = xlApp.Run("'" & xlWB.Name & "'!MacroName", strID)

    Set fso = CreateObject("Scripting.FileSystemObject")
    If fso.FileExists(strPDF) Then
        Set olReply = olItem.Reply
        With olReply
            .Attachments.Add strPDF
            .Display
            '.Send 'Restore this line after testing
        End With
Debug.Print strID & vbTab & strPDF
    Else
        MsgBox "The file" & vbCr & strPDF & vbCr & "does not exist."
    End With
lbl_Exit:
    xlWB.Close 0
    If bStarted Then
        xlApp.Quit
    End If
        Set xlApp = Nothing
        Set xlWB = Nothing
    Exit Sub
End Sub

Sub TestMacro()
Dim olMsg As MailItem
    On Error Resume Next
    Set olMsg = ActiveExplorer.Selection.Item(1)
    xlGetData olMsg
lbl_Exit:
    Exit Sub
End Sub