I'd like to make my app to open a specified PDF by an external app of the user's choice on the iPad. How can I do that? Or, is there any open-source PDF reader framework available so that I can put it into my app?

My situation in more detail:

I'm thinking of porting to the iPad from OS X / rewriting from scratch for the iPad an app which manages lots of PDFs (journal articles, etc.), but I don't want to write the PDF reader part, because there are many good ones already out there; I don't want to reinvent the wheels. (You might say you shouldn't reinvent pdf management apps, but I'd like to make one as a front end to SPIRES, and there isn't one so far.)

As the app would be a front end to a serious reading activity, UIWebView's pdf capability is not enough. Also, users of my app would have various preferences which app to use.

That's the background behind my question. Thanks in advance!