With iOS 9, a new tool lets you easily mark up a document from within Mail, re-attach it to the email, and send it back in the same message thread.
To begin marking up an email attachment, perform the following steps.
1 Open the Mail app and select a message containing an image or PDF document.
2 Tap and hold the attachment you wish to mark up.
3 Select Markup And Reply from the action sheet that appears.
4 Tap and hold the attachment you wish to mark up, and select the Markup button to open the editor.
With the Markup editor open, you can change color of the drawing, alter the thickness of the lines, add callouts by drawing with your finger, add text by tapping the text box button, or add your signature by tapping the Signature button.
When you are done marking up the document, you can tap Done to attach the marked up item back to the email reply or, you can tap Cancel to quit without saving the edited changes.