Search for one letter only In Apple apps with a search feature, how do I only search for one letter?

Let's say I'm reading a math paper, but forgot what the variable "s" meant. In Windows/Linux apps I could just include spaces in my search (" s ") and it found all instances of the lone letter "s". This does not work with my Apple apps and a search for " s " returns all occurrences of the letter "s", ignoring whether the "s" is part of a word or not.

Thanks!

Ps. the specific program I'm using is "Preview".
You quote your search string (put the quotes in the query)

" s "


to force Preview to match the exact query