Today, I read an article about how to read literature, of which the author told his experience that he marked the read papers on the pdf file, then he could know next time that papers had been read.  

Then I download one pdf editor (Foxit  Pdf  Reader)from the following sites http://www.skycn.com/soft/20490.html and had a try. It's really very convenient!

You can mark any part of the pdf file, you can draw a line under one phrase or a sentence to highlight them, or draw any other kinds of lines or graphs as you like ! (see illustration in the below picture!)

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Foxit  Pdf  Reader，是一个小巧的PDF文件阅读器，你可以方便地将PDF文件直截拖放进活动窗口打开，并支持打印，文本查看等功能，有了它您就不要为打印一篇PDF而大动干戈，本版为最新3.0版本的免费汉化版本。3.0版本增加了下列新的功能:
1.书签编辑
2.多Tab页同时浏览多个文件
3.多媒体播放支持
4.支持添加注释