title: How to insert invisible text into a PDF?
body: <p>UPDATE: Please see <a href="https://softwarerecs.stackexchange.com/questions/71464/java-library-to-insert-invisible-text-into-a-pdf">https://softwarerecs.stackexchange.com/questions/71464/java-library-to-insert-invisible-text-into-a-pdf</a> instead.</p>
  
  <p>I want to insert invisible text into an existing PDF file, to make it searchable.</p>
  
  <p><strong>What library</strong> should I use?<br>
  I would appreciate links to specific <strong>API methods</strong> to use.</p>
  
  <p>Free, ideally open source.<br>
  Thanks a lot!</p>
  
  <p><i>(For the curious: I want to automatically OCR incoming scanned papers and make them searcheable, in an Alfresco repository)</i></p>