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iOS has many levels of sandboxing, so if you were to download files using a specific application, you can only view that downloaded file in the application that was used to download that file. If you used a browser such as Mercury, there's an option to view your downloaded files by selecting it in the downloads section of the browser, and other apps will have the same thing conceptually. Unless your device is jailbroken, you cannot manipulate the raw system file/directories like you would be able to in say a Windows PC by going to C:\Program Files\ and so on.
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Files don't "download" to the iPad (there is no central storage). What happens is you select which app can/will open a file and then view that file in that app. Usually, the file is stored within the app's file system - so try to find a specific file in the app you used to view it.
Do note that the Mail app has a quick view function so you can look at a file within Mail without having to open another app. If that's where you got/saw the file, it will still be in the Mail app as an attachment (it did not download).
The iPad is app-centric, not file-centric. This is a security feature of Apple's.
Hope this helped. If not, holler back and we'll take another shot at it.
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