From Pathfinder Lost Omens Tian Xia Character Guide, which isn't enabled.

Flatten

FAMILIAR ABILITY

Your familiar can flatten itself to the width of a sheet of paper, easily fitting through the smallest of cracks and gaps. So long as a door or other obstacle has a gap that can fit a sheet of paper, your familiar can move through it without needing to Squeeze.

Pathfinder Lost Omens Tian Xia Character GuideORC