Lingoes 灵格斯词霸

http://www.lingoes.cn/zh/translator/trans_downsoft.php?id=61
Found a very good dictionary here! Lingoes: 灵格斯词霸 2.3.0 (英汉词典简体版)
it's a free software, .... with some dictionaries intalled together: 维科英汉 / 汉英词典 (简)互动百科 + Google网络释义
you can search english words using chinese words or vice versa....

After that, you can install some other dictionaries in the following url...
http://www.lingoes.cn/zh/dictionary/index.html
(or I should say, you must install the Lingoes software first, before you want to install these dictionaries..)
These different dictionaries help to explain the words better...

You can find the meaning of the words in the website/WORDS etc... by CTRL + right click mouse (of self reset the hotkey)
For the case of PDF, I tried but failed... finally, I knew that I must first install the Adobe Acrobat/Reader Plug-in, this can be found in Configuration of Lingoes. Then, while reading the pdf file, you can CTRL + C to find the meaning of the words.....(I tried and it's successful, maybe different version cannot?)...

it's really useful! haha