Re: error analysis—it's a "picture of line-art" kind of document, isn't it? Not even a watermark. So: scan it at super high resolution, blow out the contrast, countour-vectorize it to SVG (effectively equivalent to recovering the original PDF), edit that, print the result, and then scan the print.

(It just occurred to me that if there was "an app for that" in this case, no parent would ever be able to trust their child's report card again.)