If more countries start participating in PDAS will I need to go back into the service to add these countries or would these countries automatically have access to the priority documents already on the system?

Any countries subsequently joining PDAS will not be automatically selected. Therefore, you will have to go back into the service and add the new countries to those already selected. You may also deselect any country previously selected.

The process of selecting and deselecting countries may become easier if/when the proposed "registered user" system is implemented by WIPO. This system would allow you to set up your own profile of countries.