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

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