On a side note: if you copy the word "difficulty" from the Tor whitepaper and paste it into a text editor you'll get the "di_culty" as a result. As far as I understand, "fi" letters combination is badly interpreted by latex compiler and is replaced in PDF by a special character that looks almost the same as these two letters but can't be copied.
The "special character" you are trying to name is called "ligature"
http://en.wikipedia.org/wiki/Typographic_ligature .
TeX certainly has the capability to take certain letter strings, e.g. "ffi" and replace them with the traditional-typography ligatures "ffi". The native TeX fonts (Computer Modern) all have ligatures, but the tools that post-process them to other fonts (like Adobe Type 1) for PDF formatting may be missing or mishandling the ligatures.
In my browser and in my text font the single character "ffi" "ffi" ligature displays just fine. So this maybe something to do with your configuration if you can't see the "special characters".