FIND command can scan large files line-by-line to find a certain string. It doesn't support wildcards in the search string.

find /i "Completed" "%userprofile%\Downloads\*.log" >> %targetdir%\tested.log

TYPE scan2.txt | FIND "Failed" /c && echo Scan failed || echo Scan Succeeded

FINDSTR command is more feature reach, and supports Regular Expressions (REGEX) search with wildcards in the search string.

FINDSTR /L /C:"Completed" Results.txt

echo %%G | findstr /r /b /c:"[ ]*staff.*" >nul && echo Found!

See FIND and FINDSTR help sources for more information.

The following script shows more advanced split file technique, where FOR function loops through a list of files in a directory, and each file content is piped to FINDSTR that looks for a string containing substring var preceded by undefined number of spaces and superseded by any extra text. Once found, the searched file is replaced with a new one that contains only the text portion above the search string.

@echo off
setlocal enabledelayedexpansion
pushd "%temp%\Test"
for %%G in ("*.txt") do (set "break="
    (for /f "tokens=*" %%H in (%%~G) do (
        if not defined break (
            echo %%H | findstr /r /b /c:"[ ]*var.*" >nul && set break=TRUE || echo %%H )
    )) >> %%~nG_mod.txt
    del %%~G & ren %%~nG_mod.txt %%G )
popd
exit /b

Note, how setting break=TRUE allows to exit FOR loop from the file searched, once the 1st occurrence of the search string is found.