Hi @bhanu chandaluri
By default, PDF previews are not turned on in the search core results web part. Office Web Apps only supports Office documents such Word, Excel, PowerPoint, ad OneNote, but there is no support to preview other documents such as PDF.
To enable PDF preview, you could create a custom PDF hover display template or use word hover display template, for more information, you could refer to:
https://www.maadarani.com/enable-pdf-preview-sharepoint-search-result/
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