this depends on so many factors so there is no regular answer.
Basically you can save you file as Optimized PDF from the file menu.
This can reduce your file size by keeping the quality.
If your file contains images, embedded fonts, metadata, vector images, attachments, scripts etc. it is more difficult to reduche the size.
Best way is using the Preflight Pulg In (In Acrobat X under Tools > Document Processing > Preflight).
It has a large amount of fixups that can use reduce the size.