To do this you need to read technical description of archive format, find the signature archive begins with and remove all bytes lying between the beginning of file and this signature. You could also find this signature by looking at first bytes of normal archive. Passwords of any length can be cracked instantly. But because of US export regulations Office 97 uses 40-bit key. So encrypted files can be decrypted without password knowledge in a few days by searching for this key.
Some companies offer such a service.
But if your files are exclusively confidential, you may prefer do key searching by yourself. These methods will test all simple passwords. The standard security provided by PDF consists of two different methods and two different passwords. The second method is insecure and can be cracked instantly, the first one is better, but key length is 40 bit that also results in success of key-searching procedure . OS passwords 3.1.