Просто установите имя файла явно.
Sub Silent_save_to_PDF()
Dim extra_text As String
Dim file_name As String
extra_text = "123"
file_name = ActiveDocument.Path & "\" & Left(ActiveDocument.Name, InStrRev(ActiveDocument.Name, ".") - 1) & extra_text & ".pdf"
ActiveDocument.ExportAsFixedFormat OutputFileName:=file_name, _
ExportFormat:=wdExportFormatPDF, OpenAfterExport:=False, OptimizeFor:= _
wdExportOptimizeForPrint, Range:=wdExportAllDocument, Item:= _
wdExportDocumentContent, IncludeDocProps:=False, KeepIRM:=True, _
CreateBookmarks:=wdExportCreateNoBookmarks, DocStructureTags:=True, _
BitmapMissingFonts:=True, UseISO19005_1:=False
End Sub