说明：  仅做参考，为了能学习到更多的东西，做一个形式，具体是什么，我也不清楚，貌似是一个能用来看pdf的软件。。。吧
(Just for reference... I even do not know what it is, maybe a software to read PDF files. I am a student learning CT image reconstruction now, so I chose a label that is just familiar.)