@echo off
setlocal enabledelayedexpansion

for /l %%i in (1,1,10) do (
    set "filename=file%%i.txt"
    set "content="

    for /l %%j in (1,1,1024) do (
        set /a "randomnum=!random! %% 26"
        for %%k in (!randomnum!) do set "content=!content!!abcdefghijklmnopqrstuvwxyz:~%%k,1!"
    )

    echo !content! > !filename!
)

endlocal