Embed Font and Measure text

[helderdias15]

Hello!! I'm a student and i have been working with pdf-lib for my project.
For the final part of download a pdf document, i append a black rectangle for the confidential text that i select
but, happens that even with this rectangle, still possible to select all the text and see the content behind the rectangle.
i saw that in this pdf https://github.com/Hopding/pdf-lib/blob/master/assets/pdfs/examples/embed_font_and_measure_text.pdf
the content when we try to copy select is encrypted, but i can't understand how to do it.
Can you please share with me a solution of how to generate a pdf like this one to be able to encrypt the content inside the black rectangle. Thanks You.