Make a virus for Android phones which, after infecting most phones in the world, displays the "Hello world" notification on all of them simultaneously.
You have to write the message onto the moon, so it needs a lot of infrastructure but the program would be in G-Code.
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> most people as possible read it
You said "most" and not all. So the best was to reach most people would be to do what tech or media companies do. Create a viral campaign and entice news media with click-baity headlines so that they get more views and make money. You will have to translate the "Hello World" sentence into local languages for maximum reach.
Also, you did not say to do this cheaply. So for people with no internet, I would fly air balloons with massive "Hello World" banners.