The action you want to use is;
Custom Actions > Part #1 > LoadFile
Set it to the filename you want to open (including path obviously). The "common dialog" extension is useful here.
eg.
1. button --- is clicked
-> common dialog --- show open dialog
2. common dialog --- open dialog is ok
-> activex --- loadfile(filename=GetFilename$( "Common Dialog object" ))
So far as I'm aware, there's not really anything you can do with the file once it's open - you can't edit it or extact strings from it or anything like that.
I'm wanting to create an autorun for a cd i'm making which lists all the pdf files using tree control or similar and then when you click on the file name it displays it in the frame.
I don't know what DotNet is ... but maybe you should try AutoHotKey.
AutoHotKey is an awesome scripting program that you can create Automatic actions with.
I've used it a few times when I played Kingdom of Drakkar for things like dragging all items in sack to the shop for selling quickly, grabbing all of a certain kind of item and putting in sack, opening KoD if it isn't already opened, logging back in if lag or some other reason booted me out of the game, fighting enemies without being there (careful if you try that, I got caught a few times cause I was too lazy to make a more intelligent script) ... but, if you got good, you could script it so when talk windows pop up, it would answer with something vague, (a good way to keep sysops off your back) (technically scripting was against the rules).
But, yea .. AutoHotKey is powerful, you can even make scripts to turn your joystick into a mouse, or make a normally keyboard only game run with a gamepad.
Like anything though, it has its quirks, and it takes time to figure out, but there's a community of help for it.
If there's no way even with that ... then I guess a coding language is your only hope ? C++ ? Java ? .. isn't Java the one for making Active X's ? .. <shrugs>
Blood of the Ancient One, Seen only as Shadow, Faster than Lightning, Fierce as the Greatest Dragon, Nearly Invisible, Floating in a Dream, Entered through the Demon Door, Destroyer of Evil in a Realm with a Red Sky Scarred, Who could I be ?