Programatically Design Word Document using spire.doc.

You can use LEADTOOLS HTML5/JS Document Editor SDK technology in your application.
https://www.leadtools.com/sdk/document/document-editor-html5

You can leverage the DocumentEditor object, which will allow you to load or create and edit Microsoft Word Documents(DOC/DOCX), as well as PDF, RTF, and more, in an ASP.NET application. Also, LEADTOOLS does not require the MS Office interop files to work with MS Office files.

I was able to find a step by step HTML5/JS Document Editor tutorial in the link below:
https://www.leadtools.com/help/sdk/v22/tutorials/html5-import-edit-save-douments-with-the-document-editor.html

How do you want to design the Word document? Inserting table of contents? If so, you can check the following tutorial:

https://www.e-iceblue.com/Tutorials/Spire.Doc/Spire.Doc-Program-Guide/Page-Setup/How-to-create-Table-of-Contents-TOC-in-C.html

If this is not you want, you can check other functions through the following link:

https://www.e-iceblue.com/Tutorials/Spire.Doc/Spire.Doc-Program-Guide/Spire.Doc-Program-Guide-Content.html

You can also post your requirement on the support forum of Spire to ask for demos:

https://www.e-iceblue.com/forum

Hi,

Please check the following reference :

https://www.c-sharpcorner.com/UploadFile/2a6dc5/convert-pdf-to-other-format-using-spire-pdf/