I need to visualize a document attached on a pdf file on an iPad. Is there any application to do that ?

Yes. Quite a few actually.

Starting with Apple's own iBooks App, I also see many recomendations for Goodreader, and Adobe's own PDF Reader. Just to name a few. Look for them in the App Store. iBooks and Adobe reade are free. You can also search the App Store for other alternatives.

Also for most PDF's the Mail App can open them directly.