Another technique is variation of language (synonyms), orthotypography, variations in the white space of the document... so you can take a blurry B&W picture but they still got you because your version was the one saying "amazing" and not "fantastic" in the third paragraph and using the Oxford comma in the fourth paragraph, etc.
One I vaguely remember reading about years ago (not as an example, as an actual instance) included where the word wrap occurred, so on some copies a word ended a line and on others the word started the next line.
