How can i make some changes to PDF document?
You have to use an editor like Adobe Acrobat Professional which is quite expensive or Foxit PDF Editor is a cheaper alternative (a free six month license is available). There is also a free open source editor available through sourceforge. Foxit PDF Editor is the first and still the only REAL editor for PDF files, which allows you to easily and efficiently change contents in any existing PDF files. Another method (which is free) would be to extract the text and use a word processor like Microsoft Word or Open Office (free) to edit your text. I recommend PDFTextOnline. It’s easy to use, free and works from your browser.
