This species is characterized by a wide head with a flat bottom, and very large sensory canals along the lower jaw.

Crampton described P. amazonensis in 2006, in the scientific journal Ichthyological Exploration of Freshwaters; the type specimen is a 12.1 cm (4.8 in) long fish caught near the municipality of Alvarães in the Brazilian state of Amazonas.

From morphological similarities, P. amazonensis is thought to belong to the navajini, an informal taxonomic group of apteronotids adapted to fast-flowing, large rivers, with the genus Sternarchella being its closest relatives.

Virtually unpigmented aside from tiny chromatophores speckling the bottom of the head and branchiostegal membranes, P. amazonensis is uniformly white-pink with translucent fins.

[1] The highly compressed form, small eyes, and reduced scales and coloration of P. amazonensis are consistent with living in deep (up to 25 m), fast-moving waters in large rivers.