It's visible in the PDF. I have used that extension to mark draft versions of documents. This makes it very obvious and saves you from accidentally handing in a draft. At least back when things were printed out much more often. With PDFs I find that the file name is sufficient.

Does PDF actually allow some objects to be invisible on screen but visible on print? Because that'd be cool.

It's 2225. Archaeologists discover yet another long forgotten university library storage facility. Inside, they find stacks of Elsevier journals that have never been opened. They then find puzzling coffee stains that somehow appear to be result of the printing process, and conclude that the cultural significance of these markings was probably lost to the ages.

you chose where to place it:
https://ctan.mirror.garr.it/mirrors/ctan/graphics/pgf/contrib/coffeestains/coffeestains-en.pdf

That's probably where line breaks were at some point, and some garbage formatting leaked in when moving the text.

Pratchett said the insanity of person can be judged by an excessive use of exclamation points, yet he haven't lived long enough to see the pure morbidity of someone who frequently uses double spaces and, coincidentially, contributes to LaTeX.

It looks like it's typographically aligned "justified", meaning it has a standard width and will automatically add gaps between words to make it fit the page. It looks cleaner and used in many legal and academic documents.

Latex formats lines like newspapers where the left and right side of the text box make clean vertical lines and the width of a space isn't guaranteed, which might be what you're referring to. Actually, I'm pretty sure accidentally typing a double space is impossible, latex will remove them and reformat the words as it sees fit.