Best format for keeping email ephemera?

In my job i need to save copies of invitations sent via email as they constitute 'ephemera'. The invitations are not attachments but actually in the body of the email itself. I'm really hoping to get some advice as to what format i should save them in. Is it best to simply save them as HTML or to convert them to something like PDF? Even a short term solution would be most helpful as they are starting to mount up! Many thanks.