互联网下载
从互联网下载文档是很方便因为我不必须离开家。
我能发现一切用搜寻引擎 (用搜索引擎发现一切)进而减少时间下载（下载？？ do you mean to reduce the time to download stuff?）。
不然，我必须出去商店卖 (买)光盘。那是太贵啊、 （那就太贵啦！）
光盘也环境不好因为这物品是 不朽。下载和 流是环境和气。 (光盘对环境不好因为这东西不可降解，然而，下载流只是空气... i don't ready understand what you mean by "下载和 流是环境和气")
Could you post the English verison of these sentence so that I can undertand what you are saying correctly?
Here is what I learn from what you wrote:
It is convenient to download files from the Internet because I don't need to leave home.
I can find everything I needed using search engines，thus reduce my time to download stuff (?)
Otherwise, I need to go out and buy DVDs. That's quite expensive.
DVDs is bad for the enviornmnent becuase they are undegradable, however downloadstream is just air. (?)
卖: to sell
买: to buy
(I borrowed a couple corrections from Amanda because I think hers are better than mine):
Downloading files from the internet is very convenient because I do not have to leave home.
从互联网下载文档是很方便的。因为我不必须用离开家。
I can find whatever I want on internet search engines and download for free it in minutes.
我能用搜寻引擎发现一切我想要的（东西）, 而且能在几分钟之内免费下载（它们）。
免费：no cost= free
Otherwise, I would have to spend time travelling to a store and pay a high fee to buy the physical disk.
不然，我就得花时间去商店花高价买光盘。那就太贵啦！
（I used "花高价= pay a high fee", and deleted "那就太贵啦" because “高价” and" 贵"share same meaning, but you still keep both. ）
For "otherwise", you can also use “否则” 。
It is also bad for the environment, because the physical disk is made of materials such as plastic. Streaming doesn’t have any environmental impacts.
而且光盘对环境不好，因为它们是用塑料做的，然而下载对环境没有任何影响。
塑料： plastic
互联网下载
从互联网下载文件很方便，因为不(我)用出门。
使用搜索引擎我能找到一切我需要的东西，然后用几分钟的时间免费下载。
不然，我必须花时间去商店，还要花很多钱买实体光盘。
而且这样对环境也不好，因为制作光盘的材料类似塑料。然而流传输桌面技术对环境没有影响(its batter to say:然而流传输桌面技术更加环保)in China less people understand streaming .and it was the first time i heard that. I only heard desktop cloud.
互联网下载
从互联网下载文档是很方便的。因为我不必须用离开家。
我能用搜寻引擎发现一切进而减少下载时间。
不然，我必须出去商店卖买光盘。那是太贵啊！
光盘也对环境不好，因为这种物品是 不朽？？maybe is 塑料。下载不会对环境和空气造成污染。（下载和 流是环境和气。？？）
