Do you have the Hebrew font pack installed?
You can find it for your Reader version (extended languages) at http://www.adobe.com/support/downloads/product.jsp?platform=windows&product=10 (near the end of the page).
Thaks for the feedback. I have Hebrew installed on my PC, I can type in Hebrew in Outlook, is that what you mean?
Otherwise - can you please be more explicit as to what should I install?
Why don't I need anything else when using Adobe reader 9.1?
As ʇɐb ɹəuəllıʍ told you before, you need to download and install the Adobe Reader X Font Packs - extended languages
"This Font Pack supports Hebrew, Arabic, Thai, and Vietnamese as well as the Eastern and Central European languages (including Cyrillic and Greek scripts)."
As for Reader 9.1 the appropriate pack might be already/still installed on your client. It's a separate installation and if you uninstall Reader 9, that font pack remains on the client unless it's explicitly uninstalled, too.
That was the right fix, downloaded the font patch tried installing it, it told me I must have Adobe 10 reader installed first.
I did that, applied the font patch - all is fine
It left me wonder however, why I didn't have to do that with Adobe reader 9?
Maybe the appropriate font pack was already installed - and maybe you did not recognize/remember it?
So, if you install Adobe Reader 9, you have to install the extended languages pack as a separate installation - just like you did with Adobe Reader X now - to be able to read e.g. Hebrew files.
Now, if you uninstall Adobe Reader 9, the font pack remains installed on the client, unless you separately uninstall it, too.
If you reinstall Adobe Reader 9 on that client, then the font pack is still there and you're able to read Hebrew files, although you did not install the font pack again.
That's the only explanation I have.