how to convert html file to pdf using C# .net?

If you want to keep the HTML page design (including pictures and text alignment), consider convert the HTML file into raster PDF.
If you want to also keep text selectable after doing this, you could perform OCR on the result.
You can do that easily using LEADTOOLS file formats and OCR features. See this link:
https://www.leadtools.com/support/forum/posts/m41793-HTML5-Projects-with-ASP-NET#post41793