When you run the Get-FailedContentIndexDocuments cmdlet to search for items that can't be indexed by Exchange Search in a Microsoft Exchange Server 2013 environment, the messages that attach a document and the document embeds other documents are listed as a failed item. Additionally, when you use In-Place eDiscovery and Outlook Web App (OWA) to search for this kind of message, the related messages are unavailable. The extract content that's parsed from an unsearchable item looks something like this:
This issue occurs because the formats for embedding documents are parsed by IFilters. However, the .pptx, .docx, .text, .xml, .gif, .jpg, and .pdf formats aren't parsed by IFilters. Therefore, these documents are partially processed.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.