Excel to PDF .Net is a cross-platform .NET library that allows your applications to convert *.xls and *.xlsx (Excel 97 - 2019 workbook) files to PDF, RTF and DOCX formats. After adding a reference to "SautinSoft.ExcelToPdf.dll" and entering 3-4 lines of C#, you can use the API in your applications:
XLS, XLSX (Excel 97 - 2019 workbook)