A subtle new feature that’s been added to the built-in Mail app in iOS 9 allows you to mark up PDF and image file attachments. With the built-in markup editor you can add lines, shapes, arrows, and even drop in your signature. As an added bonus, if you’ve created signatures in Preview on your Mac they’ll be automatically synced via iCloud (providing you’re signed into the same iCloud account on both devices) and available to use in the markup editor.
Markup mode can be accessed by tapping and holding on a photo or PDF attachment in an email message and then choosing “Markup” from the context menu. You can even markup a PDF file or image that you’ve received in an email and automatically include it in a reply; to do this, tap and hold on the attachment in the received message to bring up the iOS Share Sheet, and then look for the “Markup and Reply” option in the bottom row of actions. You’ll be placed into the markup editor, and when you save, your marked up file will be automatically attached to a reply message in draft mode.