Because:

1. Upstream PDFMargin uses string | number
2. PDFOptions.Width / PDFOptions.Height also use string | number upstream
3. PdfOptions.Width / PdfOptions.Height using object

   puppeteer-sharp/lib/PuppeteerSharp/PdfOptions.cs

   Lines 67 to 75 in e14f59d

   	/// <summary>
   	/// Paper width, accepts values labeled with units.
   	/// </summary>
   	public object Width { get; set; }
   	/// <summary>
   	/// Paper height, accepts values labeled with units.
   	/// </summary>
   	public object Height { get; set; }

4. All of these properties were already going to ConvertPrintParameterToInches

   puppeteer-sharp/lib/PuppeteerSharp/Cdp/CdpPage.cs

   Lines 791 to 807 in e14f59d

   	else
   	{
   	if (options.Width != null)
   	{
   	paperWidth = ConvertPrintParameterToInches(options.Width);
   	}
   	if (options.Height != null)
   	{
   	paperHeight = ConvertPrintParameterToInches(options.Height);
   	}
   	}
   	var marginTop = ConvertPrintParameterToInches(options.MarginOptions.Top);
   	var marginLeft = ConvertPrintParameterToInches(options.MarginOptions.Left);
   	var marginBottom = ConvertPrintParameterToInches(options.MarginOptions.Bottom);
   	var marginRight = ConvertPrintParameterToInches(options.MarginOptions.Right);

PdfOptionsShouldBeSerializable didn't actually test Width / Height which meant that serialisation was broken by the migration to System.Text.Json.
I added a simple PrimitiveTypeConverter to support #1001

PdfOptionsShouldWorkWithMarginWithNoUnits didn't actually test the ConvertPrintParameterToInches logic so I have removed it. It was just testing what PdfOptionsShouldBeSerializable already did.

Also made MarginOptions / PaperFormat records.