Hi there,
I am just testing around with devonthink server 3. I had an issue with pdf files - in Chrome (74.0.3729.108) they were not be displayed in the preview pane. If somebody else also have this issue: Check setting “PDF documents” in Chrome (Preferences > Advanced > Site settings). If option “Download… / instead … open” is active, turn it off. For me then it worked.
Have a nice day!