It's impossible to say what percentage of files on The Pirate Bay is illegal for two reasons: 1) It's subjective. What's illegal in one country might be legal in another, and what's legal before one judge might be illegal before another. 2) files are constantly being added. For example, try and determine what percentage of videos on YouTube contain cats doing something hilarious.
However, there's a surprising amount of content on TPB that is definitely legal, Linux ISOs of various vintages, books and video in the public domain, as well as content uploaded by it's author.
There's also a lot of content of questionable legality - is a No CD patch/crack legal? After all, it's just a series of instructions to change certain bits in a file matching a specific hash. How about a keygen? That's essentially a random number generator, unless you have the program it makes keys for. Probably depends on the laws in your country.
'Fake' files are likely also legal to download, as they tend to be random data and/or uploaded by (agents of) the copyright holders.
You would be right in saying that little of that has to do with the name Pirate Bay, though.