Hi @Correy Lim ,
Yes, PDF files always be opened in SharePoint online default view. This behavior is by default. We suggest you develop your own file handler. Otherwise, it will be handled by SharePoint or if you integrate something like Adobe Document Cloud, opened by Doc Cloud (which is still in the browser, but Adobe's service instead). In this case: How to open a PDF file in directly in PDF Viewer instead of in SharePoint? ATG-5876 provided a workaround for you reference: Checkout and Open PDF files from SharePoint Document Libraries.
For Reference:
https://www.bonzai-intranet.com/blog/checkout-and-open-pdf-files-from-sharepoint-document-libraries/
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