iPad attachments

Here is a question for you: how do you, from within the email application, reply with a photo or PDF attachment? I know that you can use photos and send a pic that way, and I know that I can use good reader, but I want to know if there is a third party app that will allow you to attach files as an attachment from within the program itself? this can be frustrating especially if you don't know the email address of the person or it isnt saved as a contact.