Sometimes you can do this. There isn't a magic button for setting
size, but you can consider all of the things that contribute to the
size, such as number of fonts, number of images and other graphics,
amount of navigation. Sometimes things can be made smaller (for
example, lowering image resolution from 300 dpi to 75 dpi could reduce
space used for images by more than 90 percent). Sometimes you just
can't: for instance if you have an architects plan as lines, you can't
take out any of those lines.
If you have any more questions about Acrobat, rather than Adobe
Reader, I recommend the Acrobat Windows or Acrobat Macintosh forum.