How do i save a PDF attachment from my iphone 4s exchange email

I am trying to save a PDF attachment from my email which is on my work iphone (exchange email) so i can access it long term and also if i do not have network access at the time. I want to save the attachment as i do not have a history of all my emails on my iphone as i get heaps. I can't seem to work out how/where i can attach it. Thanks