These supplements are specifically written for Pathfinder, but I'm looking at them more from an OSR standpoint and won't be looking at game-specific mechanics. If something isn't quite right as far as Pathfinder is concerned I wouldn't know anyway. It is my belief that everything needs to be tweaked by the GM for their group anyway....
First off, the supplement comes in two downloads. One is a larger PDF tweaked for printing and the second is a smaller, lower resolution file for viewing on a screen (PC, tablet, etc.). The difference isn't noticeable unless you print the screen version instead of the print version. I do like how Raging Swan treated the print version with the same respect as the screen version. The only criticism I have with the print PDF is I really wish they would have used a different cover. Simply inverting the colors so there is a little black text on a white background is highly preferred to an all-black cover I don't want to waste ink on.
Both PDFs are well bookmarked, with some internal cross-links. The PDFs are secured, but only the ability to edit the documents, assemble a new document, or extract pages is restricted. For the majority of the end-users this is perfectly fine.
Not including things like the covers or the OGL, this supplement had 60 pages of content. There are 25 humanoid tribes detailed in two-page spreads and a nice little 5 page section of creating your own humanoid tribes. I really liked this create-your-own section, but I'm a sucker for large random tables. If you aren't wanting to go through the process of creating your own tribe there is one table filled with 50 sample tribe names. Just grab a name and fill in the background as you will.
What you are really buying with Tribes Most Foul are these 25 pre-generated tribe backgrounds. Each write-up consists of a quick introduction, society & organization, ecology & lair, combat & tactics, notable personalities, and tribal lore. You also get a good picture of a tribal totem or symbol. The overwhelming majority of the "Pathfinder Stuff" is in the stat blocks of the notable personalities. There is a bit in the knowledge checks for tribal lore as well. I really don't think that GMs for other systems will have too much trouble interpreting these stats to their system.
The more generic (system-wise) tribal information is rich with details that really make these creatures "pop". I don't think GMs will want to use this supplement "on the fly" to try and add some interest to their game, but instead plan out possible interactions and possibly work the campaign around these notable foes. There are a few integration ideas presented with each tribe.
