You do not have the option to open a .pdf file in a web browser when you click the file in a SharePoint Online library

When you click a .pdf file in a SharePoint Online library in Microsoft Office 365, you are prompted either to save the file to the local computer or to cancel the operation. The .pdf file does not open in the web browser for inline viewing. You do not have the option to open and view the .pdf file directly in the browser.

This behavior occurs because the feature to open and view .pdf files in a web browser is not available in Office 365.

To work around this behavior, save the file to a folder on the local computer, and then open it by using the Adobe Reader client.

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