Saving a booklet for digital viewing

All vector content (graphics, fonts) in a document will export to PDF in vector format and should be as sharp as the resolution of the viewing (or printing) device allows, pretty much regardless of the export settings for the PDF.

However, raster content (images) will vary considerably with that setting and any lost resolution for, say, smallest file size, cannot be recovered by viewing on a higher resolution display. It will always be a tradeoff between image file size and effective resolution/sharpness.

If your booklet has a lot of images, or large ones, it's going to generate a proportionally larger PDF. About all you can do is optimize the images for the end resolution you want to use, flatten complex graphics, and use vector graphics whenever possible. (For example, a common mistake is to have a large, type-only heading, and do it as a graphic instead of text/font over vector graphics.)

But 32 pages with images etc. — 10MB is a reasonable minimum size, just by my guess.

ETA: optimizing a doc for print and optimizing for PDF/digital are often opposed goals. You may have to create a second version, perhaps using layers, so that you have an optimal print export to PDF, and one optimal in balance between size and resolution for digital viewing.

I found the check-boxes for image quality 'High'...it's better,