Field to enter a minimum of 6 digits

I need your help on pdf
I have given a condition to the field in pdf which is when a user enters less than 6 digits, it should show the pop up notification and is written in java script and i wrote on custom validation script.

But what i want now is when a user enter less than six digits and the field have any number, it should not allow the user to move to the next field, it should allow the user to go to the other cell only when he enter 6 digits

what is happening in my pdf is when a user enter less than 6 digits it is showing the pop up notification, and not keeping the digits entered in pdf, and allowing to move to the next field so please help me on this