There are plenty of quality iOS applications for marking up documents and photos, but there's something to be said about a native solution created by Apple.
With iOS 9, users will have the ability to draw, sign and type on a document or photo attachment without having to leave the Mail app in a similar vein to that of the Markup feature found on OS X.
When viewing an email with an attachment on your iPad -- say, one asking for your signature -- long-press on the attachment and then select Markup and Reply from the share sheet. iPhone users will need to single-tap the document to preview it, then tap on the Markup icon in the lower-right corner of the screen. It looks like a toolbox.
You can then tap on the Signature icon, where you'll then be asked to create a signature using your finger and drawing on the screen. After saving your signature, place it on the document, adjusting its location and size using familiar pinch-to-zoom gestures.
You can also draw lines, circle different items or create a text box to fill in other bits of information. Additionally, there's a magnification feature for highlighting a segment of the document at hand.