It's becoming common knowledge that a file can be recovered from the hard drive even after being removed. The basic idea is that a file = a container + a content.
When you remove the file, the operating system (whether it be Windows or Linux or else) destroys the container but keeps the content. So the actual bytes of your file remain on the hard drive. And a myriad of software (most with a shareware license) have grown to sell you the idea that by writing zeroes or random patterns over the content, it will make it unrecoverable. That's theoretically true.
The problem is that the soft only destroys what you ask it to. So if there is another copy of the file, that you don't know about, that one will still be available for recovery. And that's the problem with all of MS Office software (and other office suites). These office applications create backup copies to recover if (ever) there is a crash.
And you don't ask the shredder to shred them, so they remain on the hard drive, even if you shred correctly the main file. (You can't shred them, because 1° they're necessary 2° you don't know where they are 3° that would be a long job.)
As a conclusion, if you use your shredder for office files such as .doc, .xls and so on, just drop it, it's useless.