Nobody can prevent sharing/copying of files downloaded over the Web, as computers inherently allow users to share/copy any file on them, even if they only download it once.
It's only possible to put obstacles in their way, such that they either don't know how to get around them or would probably deem it not worth the effort, so this really just helps keep honest buyers honest to deter casual file sharing.
Our PDF Stamping feature can deter most sharing of PDF files, as users are generally unwilling to share a file that contains their personal information and can be personally traced back to them.
Certainly this is not invincible; the method you describe could be one workaround, but it's so labor-intensive that realistically speaking, few if any buyers would bother investing their time and effort in such a painstaking task.
Of course, you still own the copyright on your own content, so if you find a copy circulating in the wild anywhere, you can send a formal DMCA takedown notice to the host demanding they remove the infringing content.