others have given pretty good picture of what you have to do, but you can also do this in some other language, for example in binary, or ascii, and then reduce the font size to something close to 1 pixel. the actual text of pdf is stored in seperate xml tags. Plus you can also write it simply in plain text anywhere near margin of page (no need to do color or size shenanigans) and simply crop pdf out. Cropping of pdf does not remove the stuff, just hides it. Unless you rasterise pdf afterwards and then submit, the stuff is simply there with no special amount of work required.
others have given pretty good picture of what you have to do, but you can also do this in some other language, for example in binary, or ascii, and then reduce the font size to something close to 1 pixel. the actual text of pdf is stored in seperate xml tags. Plus you can also write it simply in plain text anywhere near margin of page (no need to do color or size shenanigans) and simply crop pdf out. Cropping of pdf does not remove the stuff, just hides it. Unless you rasterise pdf afterwards and then submit, the stuff is simply there with no special amount of work required.