Upgrading from the version 9.0 to the version 10.0 we came across an issue. When we use the GetOverflowHtmlArea method and then the GetRequiredHeight method the generated pdf is wrongly formatted. If the text requires more than a single line, to be printed in the pdf, then the second line is printed more than once and this continues with the third, fourth, etc line.
Here is the C# code we use. It's almost identical to the one you have is your documentation.
Document document = new Document();
Uri uri = new Uri(@"c:\temp\temp-html-a4a33026-85a1-4afb-9d9e-7f88d9ce2504.html");
HtmlArea htmlArea = new HtmlArea(uri, 0, 0, 500, 650);
Page page = new Page(PageSize.A4);
var htmlArea2 = htmlArea.GetOverflowHtmlArea(); // <--------------
var requiredHeight = htmlArea.GetRequiredHeight(); // <--------------
page.Elements.Add(htmlArea);
document.Pages.Add(page);
document.DrawToWeb("SampleHtml.pdf");