Preview application is not showing search results when I do Cmd + F to search a string
there are multiple type of pdf encodings, some of which disable text selection and thus searching strings, which under the hood selects text and highlights text, is disabled.

check by double clicking on any word. 

1.) if the whole page becomes blue, that means you can't do searching on that page. 

2.) if the word is selected, and still you're unable to search, do comment with details, 

I'll be happy to help!