When you tap on the attachment in an email, it saves it to your phone and you'll be able to access it through the PDF Reader app. Similarly that's also the case when you download a file from the internet.
Originally Posted by goixiz
Unfortunately, and for reasons I either haven't been told or don't understand, it's not currently possible to /send/ a PDF from a Windows Phone device via email, unless you happen to have it stored on a cloud based service and have an app which can generate a link. (I get around this by keeping my PDFs on Dropbox or Skydrive)