Setting custom properties in Pdf.Generator

I am creating a pdf document  from html string using pdf.generator.pdf class. After that, I have to add some standard properties and some custom properties. With standard properties it is very easy, but I have no idea how to set custom properties.

var pdf = new Aspose.Pdf.Generator.Pdf();

pdf.ParseToPdf(html);

pdf.Creator = "My Namev";

pdf.Title = "My Title";

string mycustomkey = "my key"

string mycustomvalue = "myvalue"

???????????????? How can I set custom values here?

pdf.Save("MyPath");

I do understand that it is possible to switch to pdf.document class and use Info field, but it requires additional saving and reading from file - I want to avoid it because I am working on a big array of files.

Will appreciate any ideas

Thank you,

Alexei