Microsoft announced Edge browser in 2015 as an exclusive web browser for Windows 10, but at that time Microsoft Edge was heavily criticised as it could not perform basic functions. Microsoft acknowledged the problem and the company improved the browser with the Anniversary Update. Since their official release, Microsoft Edge is receiving updates regularly with new features, improvements, and fixes. In it’s latest February 2018 update, Windows 10 Mobile users in the production ring were unable to open any PDF file using Microsoft Edge.
It is almost impossible to think about a smartphone in which one cannot open PDF files. After receiving a considerable number of complaints from users, Microsoft has finally realized and accepted the complications and they declared that they are working on the issue, and will come up with a solution at the earliest.
Thank you for letting us know. You may encounter issues when trying to access PDF files on Microsoft Edge after the latest update on Windows 10 Mobile. We are aware of the issue and our developers are working to resolved it. Rest assured that there will be an update in an upcoming release. – Microsoft
So for now, until Microsoft comes up with the new update, you can download and install any available Microsoft Windows 10 PDF reader apps from the Store.