Change
 ' Save ActiveSheet as a PDF file to the user's Desktop and attach it to this email.    With CreateObject("Shell.Application") 
        Folder = .Namespace(0).Self.Path & "\" 
        File = Folder & ActiveWorkbook.Name 
        File = Left(File, InStrRev(File, ".")) & "pdf" 
        ActiveSheet.Copy 
        ActiveWorkbook.ExportAsFixedFormat Type:=xlTypePDF, FileName:=File 
        ActiveWorkbook.Close SaveChanges:=False 
    End With
to 
 ' Save ActiveSheet as a PDF file to the user's Desktop and attach it to this email.  
 file = ActiveWorkbook.Path & "\" & "Verduurzaming woning" & "_" & Sheets("invoice").[A2] & "_" & Sheets("invoice").[C11] & ".pdf"
        Debug.Print file
        ActiveSheet.Copy
        ActiveWorkbook.ExportAsFixedFormat Type:=xlTypePDF, Filename:=file
        ActiveWorkbook.Close SaveChanges:=False
This creates, for me, a file called "F:\Verduurzaming woning_MDMacKillop_1234.pdf"
Ensure that you have no illegal characters in your invoice number. e.g. "12/1234"