Currently, you can only set default font option for Slides as follows:
var loadOptions = new SlidesLoadOptions
Password = "secret",
DefaultFont = "Verdana" // Default font for rendering the presentation. The following font will be used if a presentation font is missing.
This functionality will be added in Words and Cells from next version of the API (18.3). As far as PDF documents are concerned, usually the font is embedded in the document. Can you please tell us, from what format you are converting to PDF?