It isn't entirely safe to install your own fonts on the iLiad, and there is not much need to do so for "Latin" languages.
The MobiPocket Reader can only use one of two existing fonts, see
Mobipocket Font Switcher. This is one of the areas where the iLiad is less capable than the Cybook, which allows user downloaded fonts in its Reader.
FBReader, like MobiPocket Reader, typically uses a single font for everything, but it can be any of the fonts installed on the device and it is possible to add new fonts for FBReader. I think PDF's can include embedded fonts, and there are several "type 1" fonts for PDFs already installed.