Converting HTML stream to PDF using Aspose – and setting FormFieldBehaviour = FormFieldBehaviour.Flattened
till generated PDF is editable when opened in browsers.

var saveOptions = new PdfSaveOptions()
{
    PageSetup =
    {
        PageLayoutOptions = PageLayoutOptions.FitToWidestContentWidth,
        AnyPage = new Page(new Margin(10, 10, 10, 10)),
    },
    Css = { MediaType = MediaType.Screen },

    // Flatten all form fields
    FormFieldBehaviour = FormFieldBehaviour.Flattened,
};

Should set PDF as non-editable.