The pdf consists of 8 pages: 1 page front cover, 1 page credits (and some text) and 1 page OGL (and some text).

It is presented in an easy to read, horizontal way and a beautiful pdf: The artworks are (with one small exception) top-notch.

The contents are:

Nightmares in the Wild: How they view their environment, planar and material.

Physiology: Really interesting read, takes not of the eyes, the flames, disguising and how they and their riders react to their flames and fires started by them.

Nightmare Sex: Reproduction and myths concerning nightmares and whether and how they can spawn tieflings.

A side box details how nightmare flesh can be made into alchemical flame bombs.

"Nightflames" and "Nightgloom" detail the gloom and flames they can release from their bodies.

"Nightmare battle tactics" is just that: Battle tactics. Efficient, intelligent, cool, battle tactics.

"Nightmares and their Riders" detail the potential relationships of the fiendish horses and their riders.

A side box details information on Vampiric Nightmares. Nice.

"Elder Nightmares" features some cool ideas on how the creatures advance and what abilities they might gain.

"Nightmares, Nightmares everywhere" deals with the connection of the horses to nightmares, i.e. bad dreams of children or omens of death for warriors.

After that, we get an elemental variety of the Nightmare, the "Karabasan". Cool critter.

Conclusion:
This entry is highly imaginative, cool, brimming with ideas and re-imagines the iconic steed of the dark knights. For the low price, we get an awesome, new take on the nightmare that lacks no associated trope or fails to address any legend while breathing life into the stereotypical "evil horse", making it something far more sinister.
Ed Greenwood and Rob McCreary have written an awesome small file that goes beyond ecology-standards.