The open behavior for documents opened from search results follows the settings of the library.
So the short answer is, set this setting to “Open in browser” via Library settings > Advanced settings should solve the problem.
For example, if you set the default open behavior to “Open in the client application” for this library, the documents will find the proper client application for interaction, Word for DOCX files and browser for PDF files, by default.
If you set this setting to “Open in the browser”, all documents (PDF included) should be opened directly in the browser by default.
If a document gets downloaded instead of opening in client applications, it is possible that the document cannot find the proper client to open. Since opening PDF files in the client application is not yet supported, make sure the PDF files are able to open in browser on your machine.
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