Since I can develop all the reports I want using XHTML, and I can transform them into XLSX and PDF, why do I need a report package again?

Using Viewerjs.org‘s viewer, I can have a clean PDF and speadsheet viewer.

With Rotativa, I can convert my HTML pages to PDF.

With the information found at C# corner, I can generate XSLS: http://www.c-sharpcorner.com/UploadFile/0b53de/simple-library-to-create-excel-and-convert-it-to-pdf-in-C-Sharp-o/

Spire is open source: http://spreadsheet.codeplex.com/ with examples: https://freenetexcel.codeplex.com/

Of course, I could use Open XML http://www.microsoft.com/en-us/download/details.aspx?id=30425