Within a PDF file text is essentially stored as single lines. The concept of text flowing from one line to another within a column does not exist. This makes it fairly difficult to edit text unless you rely on special tools that somehow manage to intelligently predict how text flows within a document
Adobe Acrobat & Acrobat Professional have a TouchUp Text tool for small text modifications. When you add or delete text, it will not reflow the text in the entire column. Some releases of Acrobat have a pretty bad reputation when it comes to the TouchUp Text tool, with users reporting weird behavior of the edited or remaining text once the tool is used. Use this tool only if there is no way to edit the original source file and recreate a PDF.
Even though Adobe Illustrator can open a surprisingly large range of PDF files, it is a bad choice if you want to edit text. Illustrator automatically uses its own fonts instead of fonts embedded in a PDF. This can lead to text reflows even if you leave text untouched.
For extensive text editing Infix PDF Editor is an interesting product. I used it to add text to a book and found the reflow capabilities of the application quite impressive.
thanks a lot...