This PCB Database is in Expedition PCB, not Pads. You must have Expedition to open it.
There is a free Expedition Browser that will let you look at the database. It can be found at:
There may be an Expedition to PADS translator available on your PADS software disk. The translator works well but not all attribute/properties are supported in both software packages. If PADS does not support the net constraint, then it will not move from Expedition to the PADS database. You might find the translator that you need at: