I took a contour plot that I had – it was a pdf generated directly from the graphing software, so it appeared to be an uncompressed image… I did the old “save as reduced size pdf” – it reduced the size by about 32%.
This is the subtraction of a cropped section of the images (the sort of compression noise that their background image could possibly have): (if the images were the same, the subtracted image would have been pure black)
much of the blockiness is just from the original image:
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