I would like to share with you that you can merge multiple PDF files into a single PDF file using Aspose.Pdf.Kit for .NET.
If you also want to convert other office documents to PDF files, in your C# application, you can try Aspose.Total for .NET product suite. Aspose.Total for .NET allows you to convert Word, Excel, Powerpoint, Text, HTML and Image files to PDF format.
I hope this helps.
Disclosure: I work as Developer Evangelist at Aspose.