- If the element that is highlighted isn't an actual image and is vector or a drop shadow/transparency, you can fix the issue by re-exporting your file from InDesign ensuring Transparency flattening setting (see below) is set to high prior exporting as a pdf
- If the highlighted element is an actual image, check the actual resolution in the file report, then up sample the image to the correct resolution (or source a higher resolution replacement) and re-export your file