I'm going to assume you have a jail broken iPad ... ?
Yes, you can open documents on an SD card in an app. However, any saved files will be in that app. As it is not normal"/stock behavior for an app to save any where else than its within its own area (unless, of course, it has the "Open In..." function to open in heat another app).
You will need to use a tweak such as iFile ($5, USD, at Cydia)to move the newly updated file back to the SD card. In fact, if I remember correctly, once you have iFile installed, you can use the app's "Open In..." Function to save it to a specific location in iFile - which can be the SD card.
Hope that clarifies. If not (or I've confused the issue too much), let us know and we'll take another shot at it.
Marilyn