Here is a nice PDF teaching Web Hacking. http://www.gironsec.com/WebHacking101.pdf

I was going through it in Google Chrome, and when I searched (Ctrl+F) for 'dos' it gave me 'ept'. Then I tried looking for other words, sometimes it returned true matches, while sometime it returned char+1 like 'q' instead of 'p'.

Then I downloaded the file and tried to open it in a text editor, but was unable to find anything useful. I converted it to .doc format, and found the letters were not matching the original letters in the PDF. I couldn't figure out more from the doc. Then I converted that doc file to PDF, and the output PDF had char+1 letters.

It would be great if someone can explain how this happens by doing some RE on the PDF.