Question

....... command is used for opening an existing document.

Answer 1

Hey !

▶<open> command is used for opening an existing document.

Answer 2

It is actually easy to open up an existing document if you are aware of the file type and location of that specific document.  

Mainly the shortcut key CTRL+O is used to open up the program so that it can open a specific file.

But for this, you must have to tap on that file before the command.