Hi Jin, is there a special reason why there are two files: File:Yunshui Jin - An Introduction to OpenGL.pdf and File:Yunshui Jin An Introduction to OpenGL.pdf ? Maybe you want to read through Help:Images to learn about the best way to upload files and avoid duplicates. You may always upload newer versions of the same file. --max 08:09, 6 August 2012 (CEST)