octet-stream

firefox突然变成了用gedit打开pdf文件，

Where the link to http://download.jw.org/files/media_m...E_20150201.pdf caused the problem of the file being offered to be opened with gedit to occur, the link to http://www.analysis.im/uploads/seminar/pdf-sample.pdf did not and offered me to open it with document viewer as expected.

The difference is the server configuration. The first file is offered with the application/octet-stream mime-type, the latter with application/pdf mime-type as it should be.
To prevent Firefox from opening every file offered with the general application/octet-stream mime-type with gedit you should alter the following files:

• ~/.local/share/applications/mimeapps.list
• ~/.config/mimeapps.list

Find the following line in those files and remove it.

application/octet-stream=gedit.desktop;

Save the file, restart Firefox and enjoy your downloads again.