For instance, if you take a copy of an article from another site, without the owner's permission, will the site suffer in Google search if they find out? What if Google doesn't find out the article is plagiarized? If no one reports it, how will they know?
I'm not sure if they check for plagiarizing, but I'm pretty sure it does something when it finds that your content isn't unique. I'm not an expert in this field though.
Possibly, they would demote the page, as you mentioned so it doesn't rank high. That's one of the Panda penalties that many article directory sites faced.
However, maybe repeated duplicate content offenses would cause Google to kick the site from any Google searches.
Definitely, this is something that site owners need to be concerned with.
Original content is always best and always rank better than duplicate content anyway.
There are a lot of signal Google uses to suspect a site of plagiarism. Before a site could benefit from the content it's posting , Google checks whether there are any similar version of the content present any where else (spin text counts too!) if not then Google will rate this piece of content based on website's authority on Google.
I can say for one thing it will screw with your page ranking, let alone Google results.
I don't think Google themselves punish people who copy things, however copying something pretty much will ruin you as Google browses your site.
It's basically googling "Taylor Swift" and seeing all the same information on sites, but they ALL have unique content with the same points, which helps. If I was to google something that was copied, I am pretty sure you and the person that you copied from would come up; which can potentially hurt the person you copied it from as well. That, and it will just lead the author of the content to you easier. 😛
It's best to have your own content rather than someone else's, because whether you copy it or not it's just not morally correct to do so. Not for your own benefit, anyway.