Once again we're on the subject of embedding fonts. Especially in Hebrew, with all its quirks, the matter of fonts is important and calls for special attention. The most familiar font type is the TTF file. This is the most common, leading format in the industry, and it's a safe bet that if you bought a font, or downloaded an interesting free font from the internet that you want to use in building your digital book, you got it in TTF format. InDesign and all the other conversion tools, like any design or editing software, let you use any TTF file you've installed on the system you work on - Windows, Mac, or Linux. When you export your book's EPUB3 file, if your fonts aren't protected and you asked to embed the fonts in the book, the font files you used will land in the FONTS directory and be linked from the appropriate CSS and XHTML files in your book. But this is the point to stop for a moment and check what the EPUB3 standard says about it, and what the recommended method is for producing a valid book that's supported by all reading applications.
An excellent technical article on the subject can be found here; it's actually part of an excellent book on creating digital books. Let's quote from it for a moment:
You may find during your testing that other font formats, such as TTF or even SVG, seem to work just fine in some reading systems. The EPUB 3 specification, however, now requires reading system support for OTF and WOFF, so why take chances on other formats? Your best option is to use OTF or WOFF.
In plain terms: the EPUB3 standard requires support for open fonts, in OTF or WOFF format. So why gamble on embedding a closed font or some other format?
Note that we've talked about the font problem before - there are ways to embed fonts and encode them differently in order to protect the font's rights. But every time we reach for a method that's a bit too "clever," we have to remember: the reading app will have to cope with it. If the user reads our book on a reading app that doesn't support the capability we're "counting on," their reading experience suffers.
The recommendation is sweeping: use open, free fonts. There are tons of them!! Even Google keeps expanding its font library, here, including support for Hebrew - a hard language. Any TTF font you have can be converted, using font software or online sites, to OTF or WOFF format - open and clean. It's simple and easy.
Using free, open fonts guarantees that any standards-compliant reading app will open the fonts correctly and show your reader the text exactly the way you designed it.