VBA to search through subfolders and open PDF based on cell value

I'm looking to be able to type a 4 digit value into cells in column M and then when I click on the cells have the relevant PDF open. The PDFs are saved in various subfolders within a folder on a shared drive. The PDF names all end with a unique 4-digit number, however they're all varying lengths and the extra bit before the 4-digit number is different for all of the them. I'm struggling to write a code for this. At the moment I've got a code which will open a PDF, but won't search through subfolders and also requires the initial string of the PDF name to be the same. This is what I've got so far:

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
'Private Sub Worksheet_BeforeDoubleClick(ByVal Target As Range, Cancel As Boolean)

    Dim fName As String
    Dim fPath As String
    Dim fullName As String
    
    'Column with pdf file names, change to suit
    Const pdfcol As String = "M"
    
    ' Bail out if something is obviously wrong
    If Target.Count > 1 Then Exit Sub
    If Target.Column <> Columns(pdfcol).Column Then Exit Sub
    If Target.Row < 2 Then Exit Sub
    If IsEmpty(Target) Then Exit Sub
    If Not IsNumeric(Target.Value) Then Exit Sub
    
'    Cancel = True 'Uncomment if using DoubleClick event
    
    'set path to file:
    fPath = "O:\01- Calculations\Reports\"
    
    'get file name from cell & adjust for formatting
    fName = ActiveCell.Value
    fName = Right(fName, 4)
    
    'add the extra 'junk'
    fName = "GBF-ZZ-ZZ-CA-S-10-" & fName & ".pdf"
    
    'try to get the full path and name from the folder
    fullName = fPath & Dir(fPath & fName)
    
    If fullName = fPath Then
        'file not found message
        MsgBox "No such filename as " & fName & vbCrLf & _
            "In Path " & fPath, vbExclamation
    Else
        'open it
        ActiveWorkbook.FollowHyperlink fullName
    End If
    
End Sub

Can anyone please help me?