Reply To: Actual PPI vs. Effective PPI

When you export to PDF, InDesign downsamples from the current image resolution (“effective ppi”) to the resolution you specify in the “Compression” pane of the Export PDF dialog box. For example, if the Effective PPI is 3500 ppi, then it will be downsampled to 300 ppi when you export the PDF (or whatever you have the compression set to).

The problem is that the Export PDF process does not downsample as well as Photoshop. So a 10 or 20% downsample is probably not a big problem. But if you downsample from 3500 ppi to 300 ppi, the image may get much softer in InDesign; Photoshop has better-quality downsampling algorithms.