Microsoft Outlook: Searching Inside PDF Attachments
Ordinarily, the installation of Adobe Acrobat Reader provides the necessary components to allow Windows' indexing system to search inside PDF files.
But on 64-bit versions of Windows, the system also needs PDF iFilter 64.
After installation, reboot to allow the Indexing Service to restart. It may be necessary to manually trigger a rebuild if Outlook still cannot see inside PDFs attached to messages.