A. 怎么把网站上的pdf文件弄出来
如果是中国知网之类的学术期刊，那么你得付费，如果只是普通的网站上的应该可以下载。如果不能下载，只可以查阅的话，我建议用截图。然后拼装。
B. pdf文件:下载pdf 文件打不开，删也删不掉
总的来说，这些PDF是个死文件，你可以将除了这些不用的文件之外的有用文件全部剪切到外面的新的文件夹，然后将死文件所在的文件夹一起删除就可以了。你需要的PDF再重新下载。
下文是别人地方拷来的，亲测有效。
My solution for this problem:
1. This kind of dead files are proced by incompletely downloading of the files from the websites.
2. When you download a pdf file from a website, rename and then save it, if the web page does not show change of the name of the pdf file, this indicates that the file have not been completely downloaded. You should immediately delete it (its size is 0 Kb.) from the folder where you save it and re-download it completely.
3. You can cut the folders in all hierarchies and files in them together or separately and paste them in other/new driver (or in new folders in new driver), since the dead files can not be moved, you can filter all dead files and re-download them according to your needs and save the in the new folders in new driver. Then you can delete the older folders in all hierarchies together with this kind of dead files completely without trouble.