Rafa, I spent a few hours today hunting answers to the same problem. According to this page, "The iFilter shell extension has a limitation with Microsoft Desktop Search and is not installed with Reader X." I'm not sure what "has a limitation with" is supposed to mean.
Adobe is really making problems for its users with this one. They stopped posting separate IFilter installers on their download page when they started bundling IFilter in with Reader (except for the 64-bit version of IFilter 9, which is available on the download page), and the installer for Reader X not only doesn't include the IFilter, but it actually removes/disables whatever IFilter you've got installed. So, you can't, for example, install Reader 9.4 to get the IFilter from it, and then install Reader X over top of it to have a more modern/secure version of Reader, because your IFilter will disappear. You are stuck choosing between Reader 9.4 with a somewhat modern IFilter and probably poor security, or Reader X with no IFilter (or the last IFilter to have its own installer, version 6.0, which is 6 years old).
Dear Adobe, this is a real mess that needs to be fixed. How many people out there have no idea that their PDF files are no longer searchable because Reader X removed their IFilter? At least create an installer for IFilter v9.4 so people can use a modern IFilter in combination with Reader X, instead of being forced to use Reader 9.4. There are a lot of posts in this forum about the missing IFilter, and I don't see a single reply from Adobe.
It's 2012. Has anything changed yet?