iOS allows you to easily read PDFs that are attached in emails, on a website or in an app like Dropbox. However, there is the Adobe Reader which is free, it stores your PDFs so you do not have to hunt through your emails and also allows you to add comments, text, highlight, underline and strikethrough text. You can draw on the PDF too. You can also add a signature.
In addition to this it saves what you have done and allows you to email it, save back to Dropbox and if you have the right printer you can print too.
In this video tutorial you will see how to do all of these things.