After you install or upgrade to Microsoft Exchange Server 2019 Cumulative Update (CU) 8 or Microsoft Exchange Server 2016 CU19, when you try to preview a .pdf document in Outlook on the Web (previously known as OWA) by single clicking on the document or by selecting Preview in the drop-down menu of the document, a download of the file is triggered. You also see a spinner instead of a preview of the documents content.
This issue is caused by a recent change made to a feature which was introduced by Microsoft Exchange Server 2019 CU7 and Microsoft Exchange Server 2016 CU18. This issue is a side effect of blocking inline images from opening in a new tab.
To work around this problem, use the Outlook client to preview .pdf documents. It is also possible to open the .pdf document, which was downloaded automatically using Outlook on the Web, with a PDF reader of your choice.
Microsoft is researching this problem and will post more information in this article when it becomes available.