I tried to "screen scrape" some PDF files once, only to find that they'd actually put the characters in the PDF in semi-random order. I guess the PDF format allows you to specify a location for each block of text, and they'd used very small blocks (smaller than a word). I suspect that the PDFs in question weren't trying to prevent screen scraping so much as they were doing something weird with their render engine.
I wonder if you could do something like that.
