In terms of content, the Abyssals feels oddly incomplete. There are plenty of details on things such as the nature of the Neverborn and the events that led up to the creation of the Abyssals, but many mechanics seems missing. For example, handling renegade Abyssals is mentioned almost constantly, but no rules for converting an Abyssal to a Solar seem present. Abyssals are supposed master of necromancy, but there are only three spells present, two of which seem reserved for the the Deathlords and one of which is specific to necrotech. Otherwise, the spells have been left for the Black Treatise. And some powers of the Abyssals mentioned in the main book, such as inflicting aggravated damage on the living, seem either missing or under developed.
In terms of artistic direction, the book is almost entirely black and white. While thematically appropriate, the result is a rather drab presentation, especially considering that colors are mentioned in the text (such as blood and rust).
The PDF itself is serviceable. There are bookmarks for each of the chapters and most of the pages render text cleanly. There are occasional areas where there are odd lines or breaks in illustrations but not many of those.
[3 of 5 Stars!]