As an addition to Brink.
Maybe you may have to check with the PDF content first.
Some PDF files contain real characters but others also have it as an image from scanned documents (means the letters in words are picture or graphic mode). And also combination of both types.
You may check it by by selecting the icon [the selection tools for text and image] do try selecting the words, if words are properly selected then your file is text, but when by the same way it turns to be a rectangle, then your file contains graphic/image.
For the latter, it's totally not possible to search any word or character in it.