How to highlight text in pdf based on textbox value

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VB

Anyone can help with how to get a text coordinates?
can this be possible?
because I just wanted a windows form app where the user types a word in a text box, and the app reads existing PDF using iTextSharp, highlights the matched words if found, and saves the PDF with highlighted text. so far i have almost everything done, including the drawing of a yellow rectangle, but what is lacking is how to get the text coordinates of the matched patterns to highlight them.

thanks in advance