This is fun. A friend of mine pioneered the "add hundreds of keywords to the PDF footer in 1-point white text to thwart keyword filters" approach that I'm sure is common knowledge now, but this is a delightful levelling-up of that venerable technique.

https://kai-greshake.de/posts/inject-my-pdf/

Inject My PDF: Prompt Injection for your Resume

To escape a deluge of generated content, companies are screening your resumes and documents using AI. But there is a way you can still stand out and get your dream job: Prompt Injection. This website allows you to inject invisible text into your PDF that will make any AI language model think you are the perfect candidate for the job. You can also use this tool to get a language model to give you an arbitrary summary of your document.

Kai's Blog
@mhoye I used to do a variation on this technique to embed an entire font in a document. In use, the document used a simple merge to add a name and a few other items to a document. The white text in the appropriate font made certain all the needed glyphs were there. The purpose was a pocket sized guide for the user.
@mhoye Search Engine Pessimization
@mhoye the puppies! Always a good subject to follow up in the resulting interview!