Currently the iPad is limited to listing 10 apps in an Open In list. To the best of my knowledge there is no way around this limit; at least on a stock iPad. There may be a jailbreak tweak for it, but that's outside my experience.
If you have apps that are clogging up that list, and you don't use. Delete them.
You can also try deleting apps you seldom use, until the app(s) you need show up in the list; then download those other apps again. They should be added onto the end of the list. Or it may work the other way around, with most recent apps showing up first. I'm not sure. It may take some experimenting. Don't forget to make sure you have good copies an apps documents before going on a delete rampage.
