I have been told that as a lawyer it is very important to use .pdf files.
Never, ever send someone a word file (or other word processor file) because for one they can alter it, and for another, all the edits and changes you made are findable either with built-in tools in Word, or with some simple forensics tools.
So the advice is always to take your carefully crafted word document and save it as a .pdf before sending it to a client or opposing counsel.
What they don't tell you is that .pdf files aren't much better. Ever wonder why your .pdf is so big? It's easy enough to search the web and find how to make them smaller. It's simple really: save them as a new file. Why? Because all of the changes you've made to the file are still there, hidden. Presumably some simple forensics could undo them and reveal the draft in which you said "and here's where I hid One-Eyed Willy's gold."
You see, when you saved, you didn't save over the old draft, you just saved changes to the end of it. The earlier stuff is still there, just hiding, waiting for some group of smart kids to find it.