PDF: get a good image quality / document weight ratio

Hello everyone!

I have a PDF problem... I'm a photographer and I have to produce a rather long PDF with a lot of images (60) almost full page. I need the images to keep an acceptable display quality, but I also need to send the PDF by email, so that it is within 5Mb. And for the moment, I'm at... 170... The PDF is intended to be opened on screen (where I need it to be in a good quality) or printed (as a technical document).

I have to admit I'm not using indesign but Publisher. But I guess it doesn't change much to my problem. For export, I tried several compression combinations but the rendering is very poor, and sometimes without even producing a 25Mo file.

The dimensions of the sheets are 254mm x 210mm and the images are about 4000 x 2500 (they are not all exactly the same size). And in the publisher's timeline, it says that the images, once embedded in the picture frames, are published at a little less than 100% (between 96 and 98, again it depends on the size of the files).

At this point, I don't see what to reduce anymore:
- the size of the sheets?
- The size of the images: I can reduce a bit, but considering the final weight of the doc, to reach 25Mb, I feel it's not realistic.
- On export, at 72 dpi I have an unusable display.

Anyway, I'm a bit lost... I'm aware that the doc is big: 60 pages of images displayed in big size. But I'm sure there's something I'm doing wrong. Does anyone have any ideas?

Many thanks in advance!!