Unless you did 'open in' on the attachment then it won't be saved in any other app - it will just be linked to the email in the Mail app, and can only be accessed via the email. Depending on the type of attachment you can download apps so that you can have access to the attachment without needint to keep the email e.g. the free iBooks app can store PDFs, as can they more powerful (but paid for) GoodReader
From the manual (downloadable from http://support.apple.com/manuals/#ipad) :
Open an attached file: Tap the attachment to download it to iPad and then open it. If iPad doesn’t support the format of an attached file, you can see the name of the file but you can’t open it.
Open an attached file with a different app: Touch and hold the attachment, then choose an app. If you don’t have any apps that can open the attachment, you can view it, provided it’s a file type that iPad supports.