You need SDK from the Android website on your Linux distro.
Power on your device tablet and you need to connect on your PC.
Enable debug mode on your tablet.
Go to shell Linux and on the platform-tools folder, you use this command:
platform-tools]# ./adb shell
Start your application from tablet and under Linux shell use the next command:
root@android:/ # screenshot /mnt/extsd/test.png
If you got some errors like error: device offline
Just go and set tablet online or from settings set to stay online.
NOTE: if you use some terminal emulator android app then the command
screenshot not working.
$ adb shell and then try