Outlook 2013 supports the inclusion of JPG, GIF, PNG and BMP image files directly in a signature. To add a PDF image, you must include it as a Web link.

How to Put a PDF in the Body of an Email. Portable Document Format (PDF) files allow users to create secure, professional documents that may include text, pictures, video and other graphics.

Some might suggest you download an app to convert a picture to a PDF on the iPhone, but the truth is you can do it for free without any apps.