Adobe produce some great software but most of it costs 'megabucks'. When they came up with the idea of the Adobe PDF format, they realized that people wouldn't pay to buy the software to create (and edit) pdf files unless the people who were to receive them could view them for free. So they created the free Acrobat Reader (which is what you're referring to as 'Acrobat 8.0') which is solely for viewing pdf files. (It's similar to the way that Microsoft provide a free program for viewing Excel files but charge a large amount of money for the actual Excel software).
To edit a pdf document, you need more than Adobe Acrobat 8.0 Reader. You need Adobe Acrobat 8.0 Standard or Adobe Acrobat 8.0 Professional. These programs cost �276.99 and �405.95 respectively! (Current Amazon prices).
There are, of course, some independent software programs which can be used to edit pdf files. Most of them still require payment but you might try PdfEdit995 8.0. It's a commercial program but the free download is fully operational with just some 'nag' screens to try to get money from you:
http://www.download.com/PdfEdit995/3000-6675_4 -10072227.html?tag=lst-1
(Save the file in Acrobat Reader, then load it into PdfEdit995 to edit it before saving your revised version).
Chris