An example of how to download bigger files via telethon, coming from ptb

bot.py

import asyncio

from telegram.ext import Updater, MessageHandler, Filters
from telethon.utils import resolve_bot_file_id, get_input_location
from telethon import TelegramClient

import logging
logging.basicConfig()


def file_handler(update, context):
    file = update.effective_message.document
    file_size = file.file_size
    file_id = file.file_id
    file_name = file.file_name
    if file_size > 20000000:
        loop = asyncio.new_event_loop()
        get_file_telethon(context.bot.token, file_id, file_name, loop)
    else:
        update.effective_message.document.get_file().download()


def get_file_telethon(bot_token, file_id, file_name, loop):
    api_id = 12345
    api_hash = "wuhuw1346789"
    bot = TelegramClient('bot', api_id, api_hash, loop=loop).start(bot_token=bot_token)
    with bot:
        document = resolve_bot_file_id(file_id)
        location = get_input_location(document)
        loop.run_until_complete(bot.download_file(location[1], file_name))
                                                        
updater = Updater("BOT_TOKEN", use_context=True)
dp = updater.dispatcher
dp.add_handler(MessageHandler(Filters.document, file_handler))
updater.start_polling()
updater.idle()

Thanks