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!