Take the image into Photoshop ( Alt / Option click with the Touchup Object tool selected ) using the Object/Image Editor tools in Acrobat PDF. These need to be set up in Acrobat preferences, make Photoshop the default. Make the adjustments in Photoshop. In this case it required: Curves to set the shadow and highlights, high pass filter and hialoam sharpening then an edge mask to sharpen. Then save, the improved image will automatically be placed back in the PDF file, with the PDF you then need to "save as" to overwrite the PDF file. You can do this with single or multiple images in the file.

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