title: Combine PDF in Rails with combine_pdf
body: <p>I'm trying to use the gem <code>combine_pdf</code> in order to combine a number of PDFs in my app. They are located in <code>assets/forms/packages/1.pdf 2.pdf 3.pdf</code>. </p>
  
  <p>I'm trying to following the gem's guide but the code yields an error:</p>
  
  <pre><code>@pdfForms = CombinePDF.new
  @pdfForms &lt;&lt; CombinePDF.new('assets/forms/packages/1.pdf')
  </code></pre>
  
  <p>I get a runtime error:</p>
  
  <pre><code>root is unknown - cannot determine if file is Encrypted
  </code></pre>
  
  <p>Any help is much appreciated, or another way to merge pdfs. The gem <code>pdf-merge</code> fails with my app due to its dependency on <code>rjb</code>.</p>
  
  <p>Thanks!</p>