You can see a preview of your message attachments right in Outlook 2016 for Mac, instead of opening each attachment in a separate application. Attachment previews are available for many file types, such as Microsoft Office documents, photos, and PDF files. You can see each page of a multi-page document, and see a collection of photo attachments as a slide show.
Open the message or view it in the reading pane.
Next to the list of attachments, click Preview or Preview All .
The first attachment is displayed in the preview window.
For more options, click Open with Preview at the top of the window, and use the controls at the top of the new preview window to interact with the attachments.
Note: To view just one attachment in a message where there are multiple attachments, select the attachment, and then click Preview or press SPACE.