You are in the right directory but the current directory is not in your shell's command search path.
Try
./adb
Also search Google for how to set $PATH variable in bash
Type the following into your terminal (from your help.ubuntu.com URL) - it will temporarily add the android tool directories into your PATH for that shell/terminal
export PATH=${PATH}:~/android-sdk-linux/tools
export PATH=${PATH}:~/android-sdk-linux/platform-tools
You should now be able to run adb (without being in the right dir and without using ./adb)
If you add those commands into your .bashrc and start a new terminal/shell you should find those changes are now permanent.