Abstract Design Pattern

I am in process of building an application that converts documents from and to different formats. For example it will covert Word, Excel to PDF. To do so I use Office API and multiple third party libraries.

For example, if Excel is installed on user's computer I will be using Excels' API otherwise will use a third party library to do the job. My problem is in implementing Abstract Factory Pattern. I added class library for each component: "MsExcel", "Excel3rdParty", "MsWord", "Word3rdParty", "PDF" and "Common". Each project references "Common" project where I plan to keep all Interfaces and each class library will implement its own component. I apologize in advance for noob question but home someone can help me because already 2nd week i am trying to make it work and no luck. Thank in advance!

public interface IOfficeFactory
{
    IExcelFactory GetExcelDocument(string documentType);
}

public class ExcelFactory : IOfficeFactory
{
    public IExcelFactory GetExcelDocument(string documentType)
    {
        switch(documentType)
        {
            case "Excel":
                return new Excel();
            case "IExcel3rdParty":
                return new IExcel3rdParty();
            default:
                throw new ApplicationException(documentType + "type can not be created");
        }
    }
}

public class IExcel3rdParty : IExcelFactory
{
    public IExcelFactory GetExcelDocument(string documentType)
    {
        return new IExcel3rdParty();
    }
}

public class Excel : IExcelFactory
{
    public IExcelFactory GetExcelDocument(string documentType)
    {
        return new Excel();
    }
}

public interface IExcelFactory
{
}

public interface IExcel : IExcelFactory
{         
}

public interface IIExcel3rdParty : IExcelFactory
{         
}