Reduced Size PDF: image resolution

[Fritz Hansen]

This is a technical question, out of curiosity about how Acrobat reduces file size—not a troubleshooting matter. 

I work on a number of projects in InDesign that involve large numbers of photographs. I export PDFs for review using a preset that reduces images (color or grayscale) to 150 dpi. Even with the reduced resolution called for in the export settings, the resulting PDF can be a very large file size, and I use the Save As Other > Reduced Size PDF command to create a smaller file for sending by email. When Acrobat reduces the size of the PDF, is there any reduction in image resolution? The photos in the reduced size PDF all look fine on screen (these review PDFs are not intended to be printed)—I don't see any degradation of image quality—but the progress bar during the reducing process typically shows image processing as a major part of the process. What's going on during the reducing process?

[Fritz Hansen]

Sorry, forgot to mention my system info:

Acrobat Pro DC continuous release 2020.006.20024

Mac OS 10.15.4

[Test Screen Name]

Yes image resolution is likely to be reduced, so is image quality. No info at all is available for what the “Reduced size” option does, leaving Adobe free to change it. If you want control use the Save as ... Optimised PDF with custom settings instead. 

[Fritz Hansen]

I may not have been clear. Saving as Reduced Size PDF does NOT appear to reduce image quality, and basically I was wondering why it doesn't. But if, as you say, no information is available for what Reduced Size does, I guess I'll never know. 

I'm familiar with the Optimized settings, but for my purposes the Reduced Size option works well and is simpler.