Renaming thousands of pdf files with different filenames

[ROSTOM]

I have thousands of pdf files in one folder and I need to rename it all. EXAMPLE, Filenames should be like this B100000_AICOR_20180209,B100001_AICOR_20180209, B100002_AICOR_20180209, B100003_AICOR_20180209.............what is the easiest way to rename all of this file?
THANKS!

[SioSio]

        Dim Folder As String = "C:\Folder\"
        Dim files As String() = System.IO.Directory.GetFiles(Folder, "*.pdf", System.IO.SearchOption.AllDirectories)
        Dim Filename As String
        Dim NewFilename As String
        Dim i As Long = 0
        For Each Filename In files
            NewFilename = "B1" & i.ToString("00000") & "_AICOR_20180209" & ".pdf"
            If Not System.IO.File.Exists(Folder & NewFilename) Then
                System.IO.File.Move(Filename, Folder & NewFilename)
                i = i + 1
            End If
        Next