FormVu renders Type 3 fonts as a graphic, preserving the display of the original page.
Type 3 fonts are not widely used, and generally we only see them used for symbol characters such as bullets or custom characters. They are often missing important encoding/mapping information that would be required to display them as a character in the web browser.
Type 3 glyphs are defined by a stream of PDF graphics commands. This means they are effectively minature PDF files and can contain shapes, bitmap images, or even other text/fonts. In practice most examples we have seen just contain a bitmap image. Web browsers do not support this type of font and so to preserve the display FormVu will render them as graphics.
If you have control over PDF creation we recommend avoiding Type 3 fonts and using an alternative option.