I assume you mean iBooks and not iPad in general...
You need to add the file com.apple.ibooks.display-options.xml in the META-INF folder with the following content:
Code:
<?xml version="1.0" encoding="UTF-8"?>
<display_options>
<platform name="*">
<option name="specified-fonts">true</option>
</platform>
</display_options>
This has been discussed/mentioned many, many, many times on this and other forums. Blame Apple.