PDF is an end destination format - although Acrobat is capable of making minor changes to a file (e.g. to correct typos) it is not a word processor or DTP application. Major changes must be done in the original document, and the PDF file re-exported.
You can copy text from a file by selecting it and right-clicking, or you can save the file as plain or formatted text. To make small changes to the text on a page, use the touchup tool - see the help file for more info.