Using GroupDocs.Conversion for .NET API you can convert a PDF document to HTML with just a few lines of code:
using (Converter converter = new Converter(@"D:/sample.pdf"))
MarkupConvertOptions options = new MarkupConvertOptions();
Furthermore, have a look at this documentation article in order to learn more about advance conversion options/features. You can also explore our open-source example project available at GitHub. Please note that this code example uses latest version (20.3) of the API.