Which version of the Arduino IDE are you using? It will be shown on the title bar of the window or on the Help > About dialog.
Please try installing it a few more times. It might be caused by a transient network outage, in which case it should work successfully after just waiting a while and trying again.
If that doesn't work, please provide some additional information:
What happens if you paste this URL into the address bar on your browser?
http://downloads.arduino.cc/tools/gcc-arm-none-eabi-7-2017-q4-major-win32-arduino1.zip
The expected result is that a 126 MB file should be downloaded. This same thing happens when you install the "Arduino Mbed OS Boards" platform via the Arduino IDE Boards Manager. The error shows that instead of a 126 MB file, the IDE is instead finding that only 947 bytes were downloaded.
So the question is: why is it only downloading 947 bytes? Is there interference from a firewall or antivirus software? Is there insufficient room on your hard drive? Etc...