what would you recommend if I have to determine performance indicators for document control?
Unless documentation has been (historically) a source of problems leading to product conformity and/or customer dissatisfaction, I would NOT attempt to devise performance indicators for document control, as this is not a core process and, in my opinion, it would amount to busy work and unnecessary bureaucracy.
On the other hand, if documentation HAS led to quality problems, we should use the actual failure modes in the document control monitoring process. I.e. If we've had nonconformities due to obsolete process specs being used, existence of obsolete documents should be a meaningful indicator to be tracked.
In other words, devising indicators that have no connection with your context makes not much sense.