I want to break up a pdf into a grid so it can not be copied. I want each page broken up into say 20 rectangles so the image on the page can not be copied when opened?
If something can be accessed or seen, it can be copied - this applies to both physical and digital works. Your suggested idea will only make it slightly harder.
Rather than go through all that trouble, I would recommend simply creating the PDF with security that prevents copying or printing. That is a readily available solution already at your disposal that would not require the added work of slicing up of your image. Combine that with a watermark layer for added security. Granted, PDF security can be easily broken with the right software, but it's more than sufficient for the average user.
Your idea doesn’t work at all because even if you cut your source image into a grid, you are then going to assemble that image into one image in your page layout app and then export as PDF. If I want to copy that image, all I have to do is do a screen capture of it with any non-Adobe PDF reader, such as the one that is built-into a Mac, iPad, or iPhone. So all you are doing is making work for yourself.
PDF already has built-in security features that prevent printing and editing. If those aren’t enough for you, then don’t publish the image. Make a print of it and put it in a safe.