Proposal for Generic Document Architecture

Document.cs

public interface IDocumentPart
{
    string Content { get; }
}

public interface IDocumentBody : IDocumentPart
{
}

public interface IDocumentHeader : IDocumentPart
{
}

public interface IDocumentFooter : IDocumentPart
{
}

public class Document
{
    public IReadOnlyCollection<IDocumentPart> Parts => _parts;
    
    private readonly List<IDocumentPart> _parts;

    public Document(IDocumentBody body)
    {
        body = body ?? throw new ArgumentNullException(nameof(body));

        _parts = new List<IDocumentPart> { body };
    }

    public bool AddPart(IDocumentPart part)
    {
        if (_parts.Exists(p => p.GetType() == part.GetType()))
            return false;

        _parts.Add(part);
        return true;
    }

    public bool TryGetPart<T>(out T part) where T : IDocumentPart
    {
        part = (T) _parts.SingleOrDefault(p => p.GetType() == typeof(T));
        return part != null;
    }
}

A document is made by its parts and that could be extended for other parts. An existence condition for document is that it needs always a body. A document can not have more than one instance of the same part.