how do i insert hidden texts that would show up when the document is photocopied?

Answer 1

You buy a special anti copy paper to do that.

(In one, the background changes to VOID, when it is photocopied.)

Answer 2

In a Word document, when you want to write a note to yourself that can be visible, or not, you:
type the note
select it to highlight
click format
cl font
about midway down, click in the box for hidden text

now you have to indicate whether or not you want to display or print hidden text:

to display it, but not print it:
click Tools
Options
View
click in the box "hidden text" to display
clear the box to hide the hidden text

To print or not print hidden text:
click Tools
Options
Print
click in the box to print
clear it to not print hidden text.