If you are simply wanting to rename any existing .PDF documents to .PDFOLD, this will be possible, however, I must caution you that there will be a problem if the target name is that of an existing file when the function is executed a second time.
For example:
thisfile.pdf is renamed to thisfile.pdfold
Later a new pdf file is created called thisfile.pdf
If the function is called again, it will try to rename thisfile.pdf to thisfile.pdfold, but thisfile.pdfold will already exist, meaning a crash and burn for the application.
What you might find as a better solution is to use a value that is incremented globally and will never be repeated. A datetime value works great in this instance. So, you might rename thisfile.pdf to thisfile_201309070337.pdfold .. this way, the next time the application is executed, the filename is incremented by the time lapsed between the last time it ran and NOW.
So, what you might want to do is this:
'filter is a string that will be used to match files i.e. use "*.pdf" to match all PDF files or "a*.pdf" to match all PDF files that start with "a"
'you can also specify certain file names or use other wildcard values like this: "??.pdf" which will find all PDF files named with two characters
'path is a fully qualified path to the location of the files that should be searched i.e. "C:\Users\User\Documents\"
'newExtension is the extension you want the files to have when the rename is done i.e. ".pdfold"
Public Sub RenameFiles(ByVal filter As String, ByVal path As String, ByVal newExtension As String)
Dim fn As String
fn = Dir(path & filter)
While (fn <> "")
nfn = Left(fn, Len(fn) - 4) & Format(Now(), "_yyyyMMddhhmmss") & newExtension
Name path & fn As path & nfn
fn = Dir()
Wend
End Sub