It is very likely at pre-pitch stage that you will have a short, concise document outlining the fundamental elements of your concept. Until fairly recently this was all that was required when pitching new products. Nowadays, however, publishers are keen to read more deeply into your game mechanics and theory.
Feature-creep has cost publishers countless millions in the past, so the more concrete a design they can see in place from the beginning, the better. A good idea is to create two separate documents: a marketing/concept overview document and a design bible.
This document is for people who do not have the time or necessity to digest every ounce of your game design, but rather need a concentrated "shot."
It should ideally weigh in at less than 20 pages, starting with a concise, accurate, and informative overview of your concept including positioning, and subsequently detailing in a similarly concise manner elements such as:
Storyline
Setting
Characters
Key features
Competitor analysis
Bullet-pointed USPs
The concept document should also be fairly graphical. Avoid filling entire pages with text; instead, try interspersing text with attractive concept and early game art.
The nature of a design document tends to vary from game to game, designer to designer, and company to company. At the pitching stage, the advice would be to focus your designers' time on documenting, explaining, and illustrating only the key functionality, including:
Control features
Game and level progression
Graphical themes
Core game systems
Any unique, unusual, or important functionality
Unlike the concept/marketing document, the design document needs only to be graphical when illustrating gameplay features.