Automatic file generating

Hello members

I'm working on an invoice document, and I would like to accomplish that when I type a number
into a table, a file is being created and saved with the number I just typed in the table.
The file that needs to be created I allready have (A template) so there is a source file.
And the files that are created can be stored in a specified location, that would be fine.)
Anyone got an idea how to do this?

Thanks in advance!