you could make a custom font with ligatures, that just replaces words and sentences with other words when rendering, and then embedd that custom font in each pdf file.
e.g. make a ligature for the word "gnome", so that it looks like "cell", and then use "gnome" everywhere in the text, where a user should see the word "cell".
edit: someone embedded doom in a true type font: https://4rh1t3ct0r7.github.io/ttf-doom/


to add insult to injure, here is AI deep research on the topic: https://assistant.kagi.com/share/8abb2344-892f-442b-9c63-09b6e72dbbab