Open the email containing the document to sign, tap on the document attachment as usual to preview it within the Mail app (the document can be PDF or otherwise) then tap the toolbox icon
Tap on the Signature button in the lower right corner of the Markup preview
Use a finger on the touch screen to sign the document as usual, then tap on “Done”
Place the digital signature into the appropriate location on the document to sign, you can resize the signature if necessary by using the blue buttons to grow or shrink the signature, then tap on “Done” when finished to insert the signed document back into the same email as a reply
Write the email reply as appropriate and tap the “Send” button to send the freshly signed document back on it’s way to the original sender
Sourced From: OSX Daily