Hi You should follow the following Steps :

1- Read the PDF file:

• Using Any tool like ->  ITextSharp , IFilter      

2- while you read the PDF File you Can change it's contents

3- Re-Generate PDF File using ITextSharp

4- Some Help links :

• http://www.codeproject.com/Articles/81118/ITextSharp-Helper-Class
• http://www.codeproject.com/Articles/12445/Converting-PDF-to-Text-in-C
• http://www.codeproject.com/Articles/13391/Using-IFilter-in-C

5- Hope this info is useful for you my friend

Sincerely,

naadydev | Senior Software Engineer