How do I set the page margins when I'm using PTConvert convertHTMLStringToPDF function?
That method uses iOS UIWebView for conversion, for which there is a limited API.
Most likely you can accomplish what you want by adjusting the CSS that is used in the HTML string.
On the server side we have a full HTML2PDF API, but this is not available on iOS.
http://www.pdftron.com/pdfnet/docs/PDFNetJava/com/pdftron/pdf/HTML2PDF.html