With December 2018 release of Acrobat DC and Acrobat Reader DC, the embedded index in the PDF is no longer used for searching.
To enable the use of embedded index to search PDFs, you have to create a registry key (Windows), or create an entry in the FeatureLockdown.plist file (macOS) as mentioned below.
Create the following entry in the FeatureLockDown file:
Lock Path: FeatureLockdown
Type: 0 (bool)
Default Value: 1
The embedded index is not used while searching when the default value of the key is 0.
To enable use of embedded index, set the value of the key to 1.