With new XRect(0, 0, page.Width, page.Height)
you specify where text will be drawn.
Use a smaller rectangle and increase the second value from line to line.
PDFsharp includes several examples:
http://pdfsharp.net/wiki/PDFsharpSamples.ashx
Especially check Text Layout. Sample code included with the source package of PDFsharp.
Also check out MigraDoc as it adds pagebreaks automatically.
http://pdfsharp.net/wiki/MigraDocSamples.ashx