PapyraSoftware 

A .NET library for composing and rendering documents with a fluent builder API. Supports output as PDF, DOCX, XLSX, and HTML from a single format-independent document model.

XlsxSharp is a .NET library for reading, manipulating and writing Excel 2007+ (.xlsx, .xlsm) files. It aims to provide an intuitive and user-friendly interface to dealing with the underlying OpenXML API.