Pran Sangli was said to have been composed presumably as an exposition by Guru Nanak Dev over questions raised by Raja Shivnabh of Sri Lanka
Tradition goes that Guru Arjan Dev when compiling the Guru Granth Sahib, despatched Bhai Piara Mokha, a learned Sikh, to Sri Lanka to bring a copy of the manuscript of Pran Sangli believed to be in the possession of the descendants of Raja Shivnabh.
The copy he brought was then scrutinized by Guru Arjan.
On reviewing the manuscript, Guru Arjan Dev understood the vulnerability of hidden essence in it and eventually decided against including it in the Granth Sahib, presumably because the subtle essence in it was only meant for devout sikhs on a higher calling than the ordinary mortals living mundane life.
Thus the teachings remained obscured for masses till date & the ripples about this holy scripture have started now when suitable time for its revelation has dawned.
Sant Rampal ji (Satlok Ashram) has a very intriguing account on it with sacred noble message from Guru Nanak Dev ji contained in holy 'Pran Sangli'.