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