@flandroid: Yeah, I have usbnet installed and ready, so at worst I can fix up/restart the Kindle manually. You shouldn't have any worse issue than the GUI being borken/unresponsive with this kind of thing, though
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@gca3020: It'll take a bit more than the Windows font viewer to know how the font is hinted, you'll have to open it with a font editor (I use Fontforge on Linux). Or with FreeType's utils if you're familiar with the different FT settings
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And even if they're not hinted, at worst they'll look a bit fuzzy (unhinted), or weird (badly hinted, than can cause broken outlines, varying weight/height across glyphs...). The only time I really had something unusable was when trying one of Vista's TTC font on my K2 a while ago, and even then, it was just garbled a bit
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So, TTC excluded, the Kindle will probably handle most of the PS/Type 1/Type 2/TTF/OTF fonts you'll throw at it
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And no, the default fonts are not user-accessible, but the full fonts hack back them up in an user-accessible directory.