The problem with modifying PDF documents is that the text of a document isn't stored "as text", that is, when one, two or a combination of bytes code each symbols of the text. Instead, it is stored as a bitmap.
Certainly, this gives an advantage in terms of potability of a document; makes it independent of computer configuration, operating system, software etc.
So, before editing a PDF document, in some cases you need to convert it first to some other format, for instance RTF, keeping the initial mark-up and formatting, and then perform editing as it is described in the article on creating PDFs. Some PDF formats allow editing PDF documents in corresponding programs without preliminary conversion.