已签了一公司的文件，这文件要求不准告公司 but ...

Most lawyers will not accept your case because you had signed the agreement.  However, you still have other way to go.

1. State legal consal

2. meadia 

3. BBB

The first two is best way for you.  State consal has power to do some actions against your employer.  Meadia such as Radio, TV station (7 on your side, etc...).

Good luck!