@echo off
setlocal enabledelayedexpansion
for /l %%y in (1970,1,1990) do for /l %%m in (1,1,12) do for /l %%d in (1,1,31) do (
  set pw=0%%m%%y
  set pw=0%%d!pw:~-6!
  set pw=!pw:~-8!
  pdftotext -q -upw !pw! %1
  if not errorlevel 1 goto :foundit
)
goto :eof

:foundit
echo Password is %pw%