This is a known issue with all versions of InDesign, Illustrator, and Acrobat on both Windows and MacOS. (It was reported by a major customer and I personally did the analysis of the problem internally within Adobe!)
The problem primarily occurs in InDesign and Illustrator if you dramatically downsample the base image during PDF export (InDesign) or save (Illustrator) and the image is masked in any way (either in Photoshop or in InDesign or Illustrator). The problem also occurs in Acrobat if you try to optimize PDF by downsampling such images within Acrobat with the save as optimized PDF function or even when printing from within Acrobat.
The root of the problem is that the base image is downsampled as specified, but the mask isn't. The result is a nasty image artifact.
We expect that the problem will be fixed and integrated into all three of these programs sometime within the next six months.
In the interim, the workaround is to downsample the image in Photoshop to its desired final magnification (size and resolution) before placing the image into InDesign or Illustrator. We know that this workaround is inconvenient, but it does work.
- Dov
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