Search pdf for 'eight' but not 'weight'

Select Edit > Advanced Search

Select "Match Exact word or phrase", then tick the "Whole words only" option.

Apparently this is only in Adobe Reader X (sorry).

In Adobe Reader 9 it's Edit > Search (Shift+Ctrl+F)

There's no drop down but there is the "Whole words only" option:

Most search engines will consider exact match if you surround the text with " like " eight "