Does this happen with any document, or only one specific one?
What happens when you switch to the Select tool (you may have to activate it in the menu View --> Show/Hide --> Toolbar Items --> Select & Zoom --> Select Tool ). Does the cursor change to the I-beam when you move it over text?
If the cursor remains the Arrow, your document does not consist of text, and can therefore not be searched; you would have to process it with an OCR function.
If the cursor changes to the I-beam, select some words, copy them to the clipboard, and paste them into a text editor of some kind. Compare the result with the text in the document. If you see ghibberisch, the document uses custom-encoded fonts, and you won't be able to search in it.
Finally also check the security settings: there may also be things restricting the search function.