iOS: Sign-N-Send is a helpful app that lets you sign documents (whether a PDF or an image) directly on the screen of your iOS device, allowing you to send it off by email or print it when you're finished.
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You open a file in Sign-N-Send primarily through other applications. Say you get a PDF file in Mail, you tap and hold down until you're given an option to open it in another application. Choose Sign-N-Send and you'll have the PDF open in a few seconds. From there, touching the document with a single finger will draw all over it. To zoom and pan, you need to use two fingers. This is a little finicky, but it works well enough once you get the hang of it. All you need to do is zoom in on the area you need to sign and then use a single finger (or a capacitive stylus, like the Pogo Sketch), to sign the document. When you're done, tap the green Send button up top and Sign-N-Send will generate a signed PDF for either emailing or printing. The only real downside I found with Sign-N-Send was that it wasn't optimised for the retina display. Not a problem for non-iPhone 4 users, but it seems like an app that would benefit from operating at a higher resolution.
Sign-N-Send is available now in the iTunes App Store for $2.49 or for free. Both versions have no restrictions, but the free version inserts an advertisement on the first page of every generated PDF. Both versions are hybrid apps and will work natively on your iPhone/iPod touch and iPad.