@ECHO OFF
SETLOCAL ENABLEDELAYEDEXPANSION
SET _STRING=image.pdf
SET _TEXT=.Pf
REM if delete next line
REM IF /I %_STRING:%_TEXT%=% NEQ %_STRING% (ECHO. 'not equal') ELSE (ECHO. 'equal')
REM and insert value instead of variable
IF /I %_STRING:.Pf=% NEQ %_STRING% (ECHO. 'not equal') ELSE (ECHO. 'equal')
PAUSE
GOTO :EOF
Now output is correct
'equal'