|If all files' names have the same reported structure, i.e. numbers delimited by underscores, then|
:: MYREN.BAT Usage: myren Folder_Name
for /F "tokens=1-4 delims=_" %%a in ('dir /B *.pdf') do (
ren %%a_%%b_%%c_%%d %%c.pdf
WARNING: Try on a test directory before a massive renaming as the script was not tested.