Count number of pages of pdf files in directory by VBA

I need to count the total number of pages in a big directory with thousands of pdf files.
First I need to be sure that the criteria is: pdf of the year 2020, month does no matter. In my code below you can see that I used *_2020_??.pdf what means that any sign before _2020_ may been used and after _2020_ there is again the possibility for 2 digits.

Background: naming of the pdf files is e.g. 12345_2020_11.pdf what means: name of the file is 12345 of the year 2020 of the month 11.

I found a code on the internet but it seems not to work. It stops after adding the heading titles File Name and Pages, and then there is no further progress at all or it stops without showing error message.
Conclusion after testing is: the script is ONLY looking in the root directory, not in subfolders. So I need some help to integrate code to search in all subfolders.

Sub Test()
    Dim I As Long
    Dim xRg As Range
    Dim xStr As String
    Dim xFd As FileDialog
    Dim xFdItem As Variant
    Dim xFileName As String
    Dim xFileNum As Long
    Dim RegExp As Object
    Set xFd = Application.FileDialog(msoFileDialogFolderPicker)
    If xFd.Show = -1 Then
        xFdItem = xFd.SelectedItems(1) & Application.PathSeparator
        xFileName = Dir(xFdItem & "*_2020_??.pdf", vbDirectory)
        Set xRg = Range("A1")
        Range("A:B").ClearContents
        Range("A1:B1").Font.Bold = True
        xRg = "File Name"
        xRg.Offset(0, 1) = "Pages"
        I = 2
        xStr = ""
        Do While xFileName <> ""
            Cells(I, 1) = xFileName
            Set RegExp = CreateObject("VBscript.RegExp")
            RegExp.Global = True
            RegExp.Pattern = "/Type\s*/Page[^s]"
            xFileNum = FreeFile
            Open (xFdItem & xFileName) For Binary As #xFileNum
                xStr = Space(LOF(xFileNum))
                Get #xFileNum, , xStr
            Close #xFileNum
            Cells(I, 2) = RegExp.Execute(xStr).Count
            I = I + 1
            xFileName = Dir
        Loop
        Columns("A:B").AutoFit
    End If
End Sub