Based on your code I think you will find the following should work

Sub merge()
'Sub ConvertWordsToPdfs()
Dim oDoc As Document
Dim sIndex As String
Dim fDlg As FileDialog
Dim sFolder As String
Dim sNewName As String
Dim sFileName As String
Dim lngCharacters As Long
Dim r As Range


    Set fDlg = Application.FileDialog(msoFileDialogFolderPicker)
    If fDlg.Show <> -1 Then Exit Sub


    sFolder = fDlg.SelectedItems(1)


    Do Until Right(sFolder, 1) = "\"
        sFolder = sFolder & "\"
    Loop


    sFileName = Dir(sFolder & "*.doc?")


    While sFileName <> ""
        Set oDoc = Documents.Open(FileName:=sFolder & sFileName, _
                                  ConfirmConversions:=False, ReadOnly:=False, AddToRecentFiles:=False, _
                                  PasswordDocument:="", PasswordTemplate:="", Revert:=False, _
                                  WritePasswordDocument:="", WritePasswordTemplate:="", Format:= _
                                  wdOpenFormatAuto, XMLTransform:="")
        With oDoc
            lngCharacters = .GoTo(wdGoToPage, wdGoToLast).Start
            Set r = .Range(lngCharacters - 1, .Range.End)
            r.Delete
        End With


        sIndex = InStr(sFileName, ".") + 1
        sNewName = Replace(sFileName, Mid(sFileName, sIndex), "pdf")


        oDoc.ExportAsFixedFormat OutputFileName:=sFolder & sNewName, _
                                 ExportFormat:=wdExportFormatPDF, OpenAfterExport:=False, OptimizeFor:= _
                                 wdExportOptimizeForPrint, Range:=wdExportAllDocument, From:=1, To:=1, _
                                 Item:=wdExportDocumentContent, IncludeDocProps:=True, KeepIRM:=True, _
                                 CreateBookmarks:=wdExportCreateNoBookmarks, DocStructureTags:=True, _
                                 BitmapMissingFonts:=True, UseISO19005_1:=False
        oDoc.Close 0
        sFileName = Dir()
    Wend
    Set oDoc = Nothing
    Set r = Nothing
End Sub