Normally the PAP has a duration of 10 days, with the option of extending for another 7 days. The PAP is issued by the Ministry of Home Affairs. However, the different authorities of the concerned Indian provinces can also issue the PAP, and also the Indian missions abroad. Normally the travel agent will take care of getting the PAP for the tourists.
One of the intended purposes of this policy was to protect the culture of the native people living there from the influence of outsiders. However, since it is quite easy for Indians to get an Inner Line Permit many Indians from other regions of India are moving to these regions. It is doubtful whether a large number of permanently residing Indians (from a significant different cultural background compared to the tribals) are having a lesser impact on the culture of the tribals than temporary visiting tourists.
Technically Indians with NRI status (i.e. Indians who are staying abroad for more than 6 months) are not eligible for the Inner Line Permit and therefore have to apply for a Protected Area Permit if they wish to visit the concerned areas. Of course this means that even native people of these areas have to apply for a Protected Area Permit once they become NRI. The same applies for PIO Card holders and Overseas Citizens of India.
Furthermore, there are no border controls at the protected areas' land borders, so if someone "looks native," he/she can travel undetected in the protected area even if he/she does not have proper documentation.