PDF files are generally locked and as such, you can't edit items within the basic format of the page. (Adobe Reader is just a "reader" and won't allow edits of documents..) That is unless you are using the full Adobe Acrobat program to create the page.. Unfortunately, you've not given us any information about what program you're using to make the edit..
Hope this helps and let us know more.
Hello guys, I need to edit header and footer of a PDF document. How do I do it?