How to insert invisible text into a PDF?

Question

UPDATE: Please see https://softwarerecs.stackexchange.com/questions/71464/java-library-to-insert-invisible-text-into-a-pdf instead.

I want to insert invisible text i

Answer 1

This shows how to create a PDF document containing text and this shows how to add an image. Add the text first and then add the image on top of it - the text will become 'invisible' to the end user but will remain searchable by search engines. This may also be useful.