A compelling update for SharePoint Online just in. The service will now support in-browser viewing of PDF files.
We've received a lot of feedback noting PDF viewing was limited while a number of customers expressed that they did not want PDF files to be downloadable to user’s desktops.
Clicking a PDF document stored in a SharePoint Online library....
....will now simply launch the file within your Internet browser. This too applies to PDFs embedded as attachments within list items
Read more here