I've found the following answer regarding your question. According to the second paragraph, it's almost impossible to do what you want.

Answer by user grantm on perlmonks.org:


  It may be possible - it depends
  on how the PDF was generated. If the
  option was used to convert text to
  curves then you're probably out of
  luck.
  
  Also the PDF file format has no
  concept of 'lines'. Characters in a
  particular font family, weight, size
  (etc) are placed at X/Y coordinates on
  the page. They can be placed in any
  order (ie: not just left-right, top to
  bottom).
  
  So if it were possible, one approach
  would be to find the y coordinate of
  every piece of text on the page;
  reduce that to a unique set; sort them
  and assign line numbers; go down and
  add the line numbers as text elements
  at the same y coordinate on the right
  hand side of the page.
  
  For the first part, you might find
  that the CAM::PDF module has some
  useful tools (eg: it can render just
  the text elements of a PDF page).
  Overlaying new text elements is the
  easy part. I tend to use
  PDF::Reuse but I'm sure that
  PDF::API2 could be used too.


Would it be possible to show line numbers as part of the pdf document

  Possible Duplicate:
  Is there some rule against ending a sentence with the contraction &ldquo;it&#39;s&rdquo;?  




Why is it that the following sounds incorrect:


  "Would she know where it's?"


But this sounds fine:


  "Would she know where it is?"

