All generated PDFs use the Gentium Plus font. Ideally other fonts will be added in the future, but any font would need to support italics and support all the special pronunciation marks used in the self-pronouncing Bible. Italics support is fairly common, but support for pronunciation marks, sometimes called diacritics, is less so. See one of the "Prefatory Note" PDFs under the "Self-Pronouncing" Bible area for a list of the required diacritcs. If you find a font you think supports all this please use the Contact page to send a link to it.
Although Gentium Plus supports all the diacritics, many of them would ideally be more subdued, not adding so much to the size of the letters they modify. The font does have a so-called "low-profile diacritics" feature, but it doesn't go far enough. Possibly BibleStudy.tools will modify the font to tone down the diacritics, but even if this were done permission would still be needed from the font owner, SIL International, to use the modified font.