"Most placeable candidate". It's just the insider term for the strategy I described above where you find a good candidate and then use them as bait to dangle in front of clients.
what stops these people from downloading mine or anyone's resume off LinkedIn and use that for their purposes? they wouldn't even need to bother contacting me first
Nothing. And they do already convert your PDFs to Doc for editing, often destroying part of your layout. At one job interview for a frontend developer position, I was very embarrassed to learn that the idiot recruiter had done that to my CV. My interviewer jokingly showed me how my CV looked all messed up - wrong alignment, missing icons, mixed up fonts, weird borders. All this because the recruiter wanted to remove my email address from the document...
Well, one could print the PDF on paper and then scan it as image, it will probably loose some resolution/detail, the layout will stay, unless the recruiter uses OCR, and then you will have, besides a messed layout also several typos ...