//Convert the main contents
PdfDocument doc = new PdfDocument();
HtmlToPdf.ConvertUrl("www.google.com", doc);

//Now add borders to each page with the "PDF Creator" interface
foreach (PdfPage page in doc.Pages)
{
    //The default HTML to PDF margins is 1 inch on all sides.
    //Since the box needs to contain all the main contents,
    //So here we create an AcmRender with 0.5 inch margin
    //on all sides
    AcmRender render = new AcmRender(
        page, 0, new AcmPageLayout(new AcmPadding(0.5f)));

    //Create an AcmBlock with border
    AcmBlock block = new AcmBlock();
    block.Style.Border = new AcmBorder(0.01f);
    block.Style.Width = 7.5f;
    block.Style.Height = 9.5f;

    //Render the block on the page
    render.Render(block);
}

//Save the result
doc.Save("result.pdf");

foreach (PdfPage page in doc.Pages)
{
    HtmlToPdf.ConvertHtml(
        "<div style='width:700px;height:900px;border:solid 1px black;'></div>", 
        page);   //Note here you pass the PdfPage, not PdfDocument object
}