title: Open a PDF file in an external app on iPad
body: <p>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?</p>
  
  <p>My situation in more detail:</p>
  
  <p>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 <a href="http://www.slac.stanford.edu/spires/hep" rel="nofollow noreferrer">SPIRES</a>, and there isn't one so far.)</p>
  
  <p>As the app would be a front end to a serious reading activity, <code>UIWebView</code>'s pdf capability is not enough.
  Also, users of my app would have various preferences which app to use. </p>
  
  <p>That's the background behind my question. Thanks in advance!</p>