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I am trying to display a Powerpoint slide deck on an android tablet, but the slides contain fonts that don't exist in android.
Is there a way to install custom fonts on the tablet? If so, how?
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In general, no. Some apps will let you add extra fonts to your SD card and search for then there, but that's app-specific.
If your device is rooted, you could replace the built in fonts with those of your choosing, but that's a system-wide change.
As mentioned in this answer, there are several apps which allow you to install custom fonts. Not all devices support this without rooting, so YMMV.
Font Installer and iFont seem like good options.
