Unfortunately You didn't supply nearly enough information for someone to know what you're trying that doesn't work.
But, I'll take a stab at it.
Try using the higher level MigraDoc library instead of PDFsharp. You should be able to take your stringbuilder, call to string on it and add that to a formatted text object.
Check out the migradoc examples and just try them out with a very large string. I suspect it will break across pages nicely, unless you have to put that large text string inside a table cell. In that case you're kinda hosed...
-Jeff