@echo off
SET FOLD=C:\TEMP\aa bb
SET XFROM=*.PDF
SET XTO=*.pdf
call :godeep "%FOLD%"
goto end

:godeep
SET SBF=%~1
echo FOLDER:"%SBF%"
pushd "%SBF%"
ren "%XFROM%" "%XTO%"
for /f "tokens=1 delims=|" %%A in ('dir /b /a:d "*.*"') do call :godeep "%SBF%\%%A"
popd
goto end
:end