compression of a pdf file to ± 5 MB

@rudid44560136 this likely is the image's resolution by lookin at the screenshot. You can save or export the PDF to be the 'smallest PDF,' but the quality is more suited for web quality. If you need a 'high-quality press quality,' then you need the higher qualty PDF. 

The question is, what is the final output of the PDF? If you need to send the PDF to a client or a commercial printer, then maybe share it through a cloud service.