BsonDocument
The BsonDocument class is LiteDB’s implementation of documents. Internally, a BsonDocument stores key-value pairs in a Dictionary<string, BsonValue>.

var customer = new BsonDocument();
customer["_id"] = ObjectId.NewObjectId();
customer["Name"] = "John Doe";
customer["CreateDate"] = DateTime.Now;
customer["Phones"] = new BsonArray { "8000-0000", "9000-000" };
customer["IsActive"] = true;
customer["IsAdmin"] = new BsonValue(true);
customer["Address"] = new BsonDocument
{
    ["Street"] = "Av. Protasio Alves"
};
customer["Address"]["Number"] = "1331";
LiteDB supports documents up to 16MB after BSON serialization.

About document field keys:

Keys are case-insensitive
Duplicate keys are not allowed
LiteDB keeps the original key order, including mapped classes. The only exception is for _id field, which will always be the first field.
About document field values:

Values can be any BSON value data type: Null, Int32, Int64, Decimal, Double, String, Embedded Document, Array, Binary, ObjectId, Guid, Boolean, DateTime, MinValue, MaxValue
When a field is indexed, the value must be less than 256 bytes after BSON serialization.
_id field cannot be: Null, MinValue or MaxValue
_id is unique indexed field, so value must be less than 256 bytes
About .NET classes

BsonValue
This class can hold any BSON data type, including null, array or document.
Has implicit constructor to all supported .NET data types
Value never changes (immutable)
RawValue property that returns internal .NET object instance
BsonArray
Supports IEnumerable<BsonValue>
Each array item can have different BSON type objects
BsonDocument
Missing fields always return BsonValue.Null value
// Testing BSON value data type
if(customer["Name"].IsString) { ... }

// Helper to get .NET type
string str = customer["Name"].AsString;
To use other .NET data types you need a custom BsonMapper class.