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.
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https://www.cnet.com/how-to/how-to-sign-markup-documents-without-leaving-the-mail-app-in-ios-9/