有些CHM文件打开后不能正常显示

需要选中该文件，然后右键选择属性，Unlock就可以了。
这通常发生在Windows XP SP2以后。
http://www.php.net /download-docs.php是这样说的：If you are using Microsoft Internet Explorer under Windows XP SP2 or later and you are going to download the documentation in CHM format, you should "unblock" the file after downloading it, by right-clicking on it and selecting the properties menu item. Then click on the 'Unblock' button. Failing to do this may lead to errors in the visualization of the file, due to a Microsoft bug.