title: R break corpus into sentences
body: <ol>
  <li><p>I have a number of PDF documents, which I have read into a corpus with library <code>tm</code>. How can one break the corpus into sentences? </p></li>
  <li><p>It can be done by reading the file with <code>readLines</code> followed by <code>sentSplit</code> from package <code>qdap</code> [*]. That function requires a dataframe. It would also would require to abandon the corpus and read all files individually. </p></li>
  <li><p>How can I pass function <code>sentSplit</code> {<code>qdap</code>} over a corpus in <code>tm</code>? Or is there a better way?.</p></li>
  </ol>
  
  <p><strong>Note:</strong> there was a function <code>sentDetect</code>  in library <code>openNLP</code>, which is now <code>Maxent_Sent_Token_Annotator</code> - the same question applies: how can this be combined with a corpus [tm]?</p>