In order for PDF files to be indexed and searchable by the SharePoint search engine, the SharePoint administrator needs to install an "iFilter" which helps SharePoint search within PDF files. This is similar to how SharePoint searches within Office files, Autodesk files, etc.
Adobe bundles a 32-bit PDF iFilter with Reader and Acrobat. That means that if you install Reader or Acrobat on a SharePoint server running 32-bit Windows, iFilter will be installed automatically. You may then need to do an additional step of registering that iFilter with SharePoint, but it’s easy to do that.
More and more users are installing SharePoint on a server running 64-bit Windows. For those users, we currently have a Labs project of a 64-bit iFilter which supports SharePoint.