Yesterday, Microsoft shipped out a monthly cumulative update for Windows 10 Mobile, helping fix devices with unresponsive touchscreens. As great as this is for the many still holding true to Windows Phone, the update also created one big problem--- Microsoft Edge no longer is able to render PDFs.
This general known issue was documented in the changelog for the update and Microsoft tells users to download the Microsoft Reader app as a temporary workaround. Though it seems like a simple solution, there is another big problem--- the app no longer works with Windows 10 Mobile.
Since Microsoft's own PDF reader app won't work in this case, we recommend using one of the many PDF readers in the Microsoft Store instead. It may be a nuisance to go out of your way and download an app, but it is the only solution at the moment.
We can also confirm the issue is persisting on our Alcatel Idol 4S running build 15254.248. Edge will get stuck in a reloading loop as it tries to render the PDF File and will eventually throw a page load error.
Since Windows 10 Mobile is no longer a high priority for Microsoft, it is likely they won't issue a fix until next month. In the meantime, you can find the download link below for a PDF reader app below.
Update: We've updated this article to clarify that Microsoft Reader no longer works on Windows 10 Mobile. Thanks Harrison for the clarification!