You'll want to use the GetExtensionName method on the FileSystemObject object.
Set x = CreateObject("scripting.filesystemobject")
WScript.Echo x.GetExtensionName("foo.pdf")
In your example, try using this
For Each objFile in colFiles
If UCase(objFSO.GetExtensionName(objFile.name)) = "PDF" Then
Wscript.Echo objFile.Name
End If
Next