It depends on what you are trying to do and how you want to do it.
If the PDF has text objects and you don’t require any advanced parsing features, you might be able to use something like the following:
Convert PDF file content into string using C#
If you want to parse the content of the document to get objects other than text such as fonts, images and hyperlinks, see the following article:
How to Read, Write and Edit PDF Files and Metadata using LEADTOOLS
If the text is in bitmap images in the PDF, you will need to perform OCR (Optical Character Recognition) on it to convert it back to text, which is explained in this article
OCR with LEADTOOLS: The Better Choice