Zosh [$2.99, iTunes Link] provides a way to do just that. It's an iPhone-based tool for marking up and signing PDF documents with text, dates, and signatures. The company says that Microsoft Word document support will be available soon.
Zosh works in concert with a secure file server to make the documents available to your iPhone. When you receive a document as an attachment to an email message, you just forward it from your Zosh account email address to a special zosh.com address. Zosh recognizes your email address and then stores the document folder; the contents of that folder are visible on your iPhone on a Documents screen. For training purposes, Zosh has created a "Getting Started with Zosh" PDF that you can read for step-by-step instructions and hands-on demonstrations.
To view the document, you simply tap on its name. When you find a place that you need to annotate with a date, text, or signature, you simply tap on the Insert button and you're given a choice of what to insert. If you choose date, a date picker with several different date formats appears. Selecting text brings up a text edit field into which you type the text you want to annotate the PDF with. To add a signature, an automatically scrolling window appears into which you write your name on the touchsc. I found that using a stylus like the Pogo Stylus helped me to create a more accurate signature than my finger.
Any of the annotations can be in a choice of four colors -- blue, black, gray, and red -- and can be moved around the page, rotated 360°, resized (bigger or smaller), or deleted. For text annotations, you can choose from a variety of fonts, although none of them really stood out as being unique. Of course, if you're annotating a contract or other legal document, I'm not sure you want to put text in some sort of oddball font.
Once you're done signing the document, you "transmit" it back to your email address or that of another recipient. There's no need to print the original, find a pen to sign the document, then scan and or fax the document before sending it on. Zosh does exactly what it sets out to do, breaking the cycle of wasteful printing and faxing, and it does it on the iPhone alone. While testing the app on a business trip, I had the opportunity to use Zosh to annotate and sign a couple of documents, and I found it to be a lifesaver.
While Zosh isn't for everybody, if you are the type of person who needs to receive, annotate, and sign documents, and then return them to another person, the app is worth much more than the $2.99 price tag.