How to search a text or a word in a pdf

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C++

Java

Android

Can someone please tell me that how does some pdf readers are able to search the text inside their pdf, I have made a reader that supports pdf, but I am not unable to search text in the pdf?

What I have tried:

I tried to use OCR and text encoding but there is a processing time required for it, further without I am unable to understand how to search for exact text inside a pdf, before converting it into any text file, it should be the same original pdf