Working with document is a process, consisting of two main parts:

1. You create or edit your document structure, describing the document definition
   1. Firstly you indicate the name and the type of the document and its factory you will later work with
   2. Secondly you define the document schema, that describes the structure of a data within the document
2. You work with corresponding document factories from your c sharp code

Every document can be described either in ServerConfig.xml file or in its own xml file using import tag. To learn how to create a document schema look at Document schema and Document attributes topics. After you have defined document structure, Workflow Server Designer will generate corresponding C sharp classes, so every document will have its own representing type. With document factories you can easily create, retrieve or delete appropriate documents. Look at this topic for some handy examples, describing how to work with documents and document factories from c sharp code. You can also look at Factories topic to learn more about different factory types.
There is also more information in Documents and Document Factories topic.

Useful topics:
Document attributes
Document schema
Documents and Document Factories
Document Factory
FAQ
Factories
Documents in csharp code
DocumentFactory class in Class Library