How to install OSX El Capitan downloaded from ftp server?
When you download the installer from the AppStore, it is saved as Install OS X El Capitan.app in the Applications folder. Behind the scene .app files are really just folders which are treated as a single file by the OS. My guess is that you either have an incomplete file or something has gone wrong when you copied the file to the ftp server. 

If you compress the .app (control+click > Compress "Install OS X El Capitan") and then copy the resulting archive to the ftp folder your other macs will be able to download the archive, uncompress it and run the app.
Or you can just copy Install OS X El Capitan.app to a USB drive or even create a bootable USB drive which you can then use as a rescue installation drive.