Depends on the format of the attachment. If it's a word document you can download Pages. Excel you need Numbers. There is also one like Powerpoint called Keynote. If it is a PDF, you can download Adobe and edit components of the document (add text, highlight, "write" in pen, sign the document, etc.) Pages/Numbers/Keynote are all $9.99 each. Adobe is free. Some people like Evernote (also free) though I'm not a big fan. You can also get One Note for the iPad for free it works just like the PC.
Moved thread to Mobile/Apps/Computing forum to elicit further response.