“the PDF document contained image masks that were not downsampled."

I need to save an InDesign document as a small PDF. The indd document is 21.5MB, and when saving the document as a PDF using the "Smallest File Size" setting, the PDF comes out to be 20.3MB. I would like to get the PDF closer to the range of 10MB-15MB.

The document in InDesign CC2015. It is 20 pages, has vector ai files, and all photos that were originally .psds have been downsampled to 72 ppi and saved as jpegs. I subsequently tried to downsize it by saving it in Adobe Acrobate Reader DC as an Optimized PDF and a Reduced Size PDF. When I do that, I always receive the following error: "the PDF document contained image masks that were not downsampled."

Can someone explain to me how I can remove the image masks, and what else I can do to reduce the file size of the PDF?

Thanks!

Mariel