Some of that may be for troubleshooting as you wrote the macro, but:
You don't need to open the Finder selection to rename the files
If you are selecting multiple files your macro will break because you are appending to LocalFileList -- the "Move" action won't receive a correct file path, even if you move it inside the "For Each", and trying to name all the selected files 123.pdf will cause problems
If you are just renaming a file but leaving it in the same place, you don't need the file path in the "to" field of the action -- just use the new name
Using a "static" name like "123.pdf" could cause problems unless you always, always, move any previously renamed files before running the macro again
All you actually need to "rename a single selected file to 123.pdf without moving it" is: Rename Selected File.kmmacros (1.2 KB)
But do have a think about what you'd eventually like to do. For example, you could "watch" your Downloads folder and any time a file whose name matched a pattern arrived you could automatically rename it -- no intervention from you. But that does mean you need a naming scheme that offers unique names (or both renames and moves the file) to prevent conflicts.