Tested on Windows Server 2019.

A complete reindex is required after this, search will be unavailable until this reindex is completed!

Install the Office 2010 Filter Pack:

1. Microsoft Download Center - Microsoft Office 2010 Filter Packs
2. Microsoft Support - Description of Office 2010 Filter Pack SP2: Microsoft Download Center - Service Pack 2 for Microsoft Office 2010 Filter Pack (KB2687447) 64-Bit Edition
3. The latest update from the Microsoft Update Catalog - Office 2010 Filter Pack

Then make sure that both .pptx and .xlsx are checked under Control Panel → Small/Large Icons view → Indexing Options → Advanced → “File Types” tab:

Source: MCB Systems - Index Microsoft Office Files for Windows Search on Server 2012