How can you hide information in a pdf?

Question

Steganography

What are the ways to hide data in a pdf file? (In addition to opening as an archive, white text on the background and in hex mode before the %PDF declaration)
And how can this data be found?

Answer 1

You can put steganography in the cover on the 1st page of "War and Peace".

And how can this data be found?

- let's say they found ...
and then what will you do with them?

Answer 2

Steganography of visual design.

Answer 3

The color shift of the page pixels cannot be detected without knowing how they were superimposed.

Answer 4

use separate container for PDF