title: combine multiple pdf plots into one file
body: <p>I am trying to merge multiple pdf plots into one master pdf file. </p>
  
  <p>Example: </p>
  
  <p>Input: I have three pdf files: "1.pdf", "2.pdf", and "3.pdf"</p>
  
  <p>Output: How to combine these three plots into one file called "combine.pdf"</p>
  
  <p>I tried using pdf(), and pdftk(), but not successful yet, may be I am missing something simply. Thought to request for help. Greatly appreciate any response.</p>