Alternative to Preview app to search for text in multiple documents at the same time
Use Spotlight.


  Spotlight is a system-wide desktop search feature of Apple's macOS and
  iOS operating systems. Spotlight is a selection-based search system,
  which creates an index of all items and files on the system. It is
  designed to allow the user to quickly locate a wide variety of items
  on the computer, including documents, pictures, music, applications,
  and System Preferences. In addition, specific words in documents and
  in web pages in a web browser's history or bookmarks can be searched.
  It also allows the user to narrow down searches with creation dates,
  modification dates, sizes, types and other attributes.


With Spotlight you can do advanced searches and filter for example on file extensions like .pdf.