Best way to compress PDF without losing quality

Thank you all for the help! 

Cheers. 

Now I can tell you why: the flower bouquet is not a pixel image, it's made of billions vectors, see it below when "Edited with Illustrator".

Pixels can be downsampled and compressed using a lossy format (JPEG).

Vectors can only be compressed without loss (ZIP) and therefore there is a low limit.

So, in InDesign you should replace this vector image by a pixel image before exporting to PDF.

Hi Zola,

I just had a chance to see your screenshot and can IMMEDIATELY see the issue: it's that dark gray background you have. Every pixel that makes up that background counts toward the total size of the document. If you were to use a white background, the size would go down exponentially. And going to a light gray would not make a difference: it's not the shade or hue, it's the fact that they exist.

On, in another way, if someone were to ask if they could use your printer to print this, and they wanted to print 100 copies, how fast would your ink supply hold up?

It's a beautiful design, if you want this to have a smaller storage size, come up with a beautiful design on white.

PS : you should write "crémeux" not cremeux.

I cannot tell you why but the problem is here:

Try the PDF Optimizer of Acrobat Pro.

Not seeing the pdfs in question, it's hard to k ow the best strategy but images are the primary problem/issue with file size. When you push compression too much, that's when quality goes down the tubes. 

Was this pdf from scanning or what?