You'll need to buy Adobe XI Pro to get an official serial number but there are lots of other PDF editors and creators that you can get much cheaper and even free. Here are a few to consider:
PDF-XChange Viewer
Download the free version of PDF Xchange Viewer to edit pdf files with lots of options to choose from. Adding comments and annotations is pretty easy and you can draw arrows, lines, ovals and boxes, colour highlight, underline and block.
http://www.tracker-software.com/product/pdf-xchange-viewer
Able Words
Able Words is a word processor that can read and write many different formats including PDF's. You are able to format tables, images and headers and footers. It also includes a nifty spell check and print preview.
http://www.ableword.net/downloads.html
Foxit Reader PDF Creator
This app allows you to add annotations, images, videos, links and other file attachments to any PDF. You can also create and insert stamps, search documents and share content with other collaborators.
http://www.foxitsoftware.com/downloads/
PDFescape
This is the go-to web app for editing and creating pdf's online wherever you are. Its fast, free and perfect for right in the moment document manipulating and sending.
http://www.pdfescape.com/open/
Skim (Mac)
For a free PDF Reader and Editor for a MAC, I have been using for free for a few years now is Skim.
http://skim-app.sourceforge.net/
Formulate Pro (Mac)
Check out Formulate Pro for another free PDF reader and editor for MAC.
https://code.google.com/p/formulatepro/
