It is possible. Add the following code to the Outlook VBA module 'ThisOutlookSession'
Option Explicit

Private Sub Application_ItemSend(ByVal Item As Object, Cancel As Boolean)
Const strPath As String = "C:\Path\ 'The path where the pdf files are stored"
Dim vSubject As Variant
Dim i As Long
    If Len(Item.Subject) - Len(Replace(Item.Subject, "-", "")) > 1 Then
        vSubject = Split(Item.Subject, "-")
        For i = LBound(vSubject) To UBound(vSubject)
            If FileExists(strPath & Trim(vSubject(i)) & ".pdf") Then
                Item.Attachments.Add strPath & Trim(vSubject(i)) & ".pdf"
            End If
        Next i
    End If
lbl_Exit:
    Exit Sub
End Sub

Private Function FileExists(filespec) As Boolean
Dim fso As Object
    Set fso = CreateObject("Scripting.FileSystemObject")
    If fso.FileExists(filespec) Then
        FileExists = True
    Else
        FileExists = False
    End If
lbl_Exit:
    Exit Function
End Function
If you then create a message with a subject that looks like 01-03-04, then provided there is at least one hyphen (and that if you have additional text it is separated from the numbers by hyphens e.g. 'Leading Text -01-03-04- Trailing text') then the subject is split at the hyphens and if there are matching PDF names to the split numbers/texts, they are added to the message when you click Send. If the PDF names don't match the numbers, you would have to use conditional statements to cross reference the names with the numbers, but this should give you the bones of a process.

01.jpg