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 "
Keep your software up-to-date, Adobe Reader 9 has bugs and security leaks.
Here is a screenshot of Adobe Reader X:
As it looks like they slightly revamped the search engine it might support the " " syntax as well...