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DPI is suspicious. Guessing both exports treated the input as 150dpi on the way in regardless of real DPI. Check the resolutions on the >1500dpi images - to convert for example 1500dpi to 150dpi without losing data you need to go up 10x in pixels, so for example 200 pixels to 2000. If it didn't you've got your culprit. PDF mid-step sounds a little like it should be removed from your process if possible. Worst-case you may even have a step along the way here that treats everything as 72dpi regardless of real input - plenty of image converters out there that ignore dpi entirely.