Most OSs show a nice summary about which application will be used to open a certain file type. Android does not seem to have such a function.
How can I track and manage the default applications?
As you maybe already know, for each application that currently is a default application for something, say a PDF viewer, you can reset this default in the application settings (Settings -> Applications -> Manage applications -> Your app -> Clear defaults). After this, the next time you perform the action (e.g. open a PDF), a popup will appear, letting you choose a new default.
As far as I know, this is all you can do with stock Android.
There are however, 3rd party apps, that give you an overview of the currently set default applications for common things, e.g. Default App Manager.
You can use Default App Manager:
It allows you to set default app for picture, music, web pages, video, etc...
If there are a lot of them, it might be easier to just clear all default applications. You can do this without installing an app or having to look through a list. Go to Settings, then Apps (in the Devices section). From the overflow menu pick Reset app preferences.
The next time you have more than one app for something, you'll get the app chooser. Any apps that you really did want as default you can set again in the same way you did before.
Default App Manager will show you all applications that have "Clear Defaults" available in the application settings screen.
After many trial and errors File Expert solved the problem. I couldn't open e-book files ("cannot open file") and programs people advised didn't fix it. Not Astro Manager nor Default App Manager. The last one only lists file types that apps REGISTERED, but if the handler would be registered correctly, I wouldn't search web for help.
File Expert gives you the option to open selected file with selected app. That what solved the problem but no other file manager does that (or at least this one the first I found does that after several app I've tried)