title: PdfReadWarning: PdfFileReader stream/file object is not in binary mode
body: <p>I have many pdf pages that I want to merge them into one file.</p>
  
  <p>My script is as follow:</p>
  
  <pre><code>from PyPDF2 import PdfFileMerger,PdfFileReader
  filename_list=[]
  merger = PdfFileMerger()
  for i in range (0,66):
      filename='page'+str(i)+'.pdf'
      if not filename in filename_list:
          filename_list.append(filename)
  for filename in filename_list:
      merger.append(PdfFileReader(open(filename),'rb'))
  merger.write('output.pdf')
  </code></pre>
  
  <p>When I ran it, it gave me the warning "PdfReadWarning: PdfFileReader stream/file object is not in binary mode. It may not be read correctly. [pdf.py:792]"</p>
  
  <p>I'm not sure why it is doing that. Can anyone have any suggestions? (I used pypdf2)</p>
  
  <p>Very much appreciated! </p>