You can easily open any attachment with another app on your iPhone, iPad (Mini), and iPod Touch. If the file is a pdf that is embedded in the email, just tap and hold on it to reveal the sharing options. This will include the option to open that attachment in another app. If the file is not embedded, like a Microsoft Word or Excel document, then simply tap on the file to open it. Then tap on the sharing button in the upper right corner of the screen. This will give you the option to open the document in another app. Tap on the app you want to open the document in.
When you use this in conjunction with an app like QuickOffice (see a review of QuickOffice in this article: How to edit Microsoft Word, Excel, and Powerpoint Documents on the iPad (Mini), iPhone, and iPod Touch), you have the ability to open Word, Excel, or Powerpoint Documents on your iPhone, iPad (Mini), or iPod Touch and edit the document. Then, you can easily save and attach the document to an email to send the edited file back to the sender.